French Onion Soup with Lentils

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This soup brings together two ingredients that are naturally rich in umami, creating a real umami bomb in one bowl. The deep caramelization of the onions, inspired by classic French onion soup, is combined with split lentils to make the dish more substantial, protein-rich, and full of flavor. It is also very easy to make, naturally vegan, gluten-free, and incredibly satisfying.

I kept mine simple because this is a soup I make often and genuinely enjoy just as it is. But if you want, you can make it more indulgent by adding toasted bread and cheese — plant-based, in my case — and finishing it in the oven, just like a classic French onion soup.

Why This French Onion Soup with Lentils Works

There is a reason French onion soup is such a classic: slowly caramelized onions are incredibly flavorful. Sweet, savory and deeply rich, they turn what could be a simple winter soup into something much more interesting.

In fact, the key to getting this soup right is really the caramelization of the onions. Once that part is done properly, everything else is very simple.

Traditional French soupe à l’oignon does not contain legumes, but lentils are a natural addition in contemporary vegetarian and plant-based versions. In this recipe, I use split red lentils, which cook directly in the soup and gradually break down, creating a thicker, velvety texture while also making the dish more substantial and naturally rich in plant-based protein.

The result sits somewhere between a classic French onion soup and a creamy lentil soup: deeply savory, comforting and completely vegan.

Ingredients for French Onion Soup with Lentils

Onions

The real stars of the dish. White or yellow onions are the most traditional choice, but I often use red onions simply because I prefer their flavor.

My advice is to use the variety that makes the most sense for you, in other words, the onions you already have at home. They will all caramelize beautifully, although the final flavor and color may vary slightly.

Carrots

I also add a small amount of carrot to the base of the soup. It is not part of a traditional French onion soup, but I like using it here as part of the soffritto because it adds a little natural sweetness and works beautifully with the caramelized onions and lentils.

Lentils

I used split red lentils, the same type commonly used for Indian dal. I like them here because they naturally break down as they cook, helping to create that thick, creamy texture without needing any cream.

They are also quite easy to digest compared with many whole lentils. If you want to experiment with another variety, I think the soup will still work well, just keep in mind that the texture and cooking time may change slightly.

Tomato Passata

Tomato is not traditionally used in French onion soup, but I added it here because it boosts the umami flavor ( tomatoes are naturally rich in umami) . If you prefer, you can leave it out and use only broth instead. I used my favorite tomato passata, from Rummo, but you can use any good-quality passata you like, including homemade.

I would not recommend replacing it with fresh tomatoes in this recipe, because passata gives the soup a smoother, creamier texture and blends much more naturally with the lentils and caramelized onions.

Vegan Butter

To caramelize the onions, I used a combination of vegan butter and olive oil. There are so many good vegan butters available now, so you can simply choose the one you like best.

If you want to explore the different options, have a look at my guide to vegan butter substitutes. And if you do not have vegan butter at home, you can use olive oil on its own, it will still work very well.

Herbs

I used dried thyme, which is also commonly used in traditional French onion soup. You can also add a bay leaf while the soup cooks for a slightly deeper, more aromatic flavor.

Broth

I used about 600 ml of homemade vegetable broth that I already had on hand, made with carrot, celery, onion and miso. But the broth is optional here, you can absolutely use hot water instead. This soup already has plenty of flavor from the caramelized onions, lentils, tomato and herbs, so a rich broth is not essential.

How to Make My French Onion Soup with Lentils

The steps for this soup are few and very simple. The only part that deserves a little attention is the caramelization of the onions, because that is where most of the flavor develops.

Below, I’ll walk you through the process step by step, but if you are in a hurry, you can skip straight to the recipe card at the bottom of the post and start cooking.

Step 1- Caramelize the Onions

Slice the onions thinly, but not too finely, and finely dice the carrots. Heat the vegan butter and olive oil in a large pot, then add the onions, carrots and thyme.

Cook over low heat for about 40 minutes, stirring occasionally, until the onions are deeply soft and caramelized and the carrots are tender. If the mixture starts to dry out or brown too quickly, add 1 tablespoon of water and continue cooking gently.

Step 2. Add the Lentils

Rinse the lentils thoroughly under running water, then add them directly to the caramelized onions. Season with salt and black pepper to taste, and stir well to combine.

Step 3. Add the Broth and Tomato Passata

Add the tomato passata and hot vegetable broth, then stir everything together. Lower the heat, cover with a lid, and let the soup simmer gently for about 20 minutes.

If you are using split red lentils, as I did, there is no need to cook the soup for much longer. The lentils will usually be tender in less than 10 minutes, but we keep them cooking a little longer so they begin to break down and naturally create the creamy texture of the soup.

Step 4. Blend Part of the Soup

This step is completely optional, but I like to blend just part of the soup to make it even creamier. Simply transfer a portion to a blender and blend until smooth, then stir it back into the pot. Do not blend the whole soup, keeping some texture makes it much more interesting. You can also skip this step entirely if you prefer a more rustic consistency.

Step 5. Serve It Your Way

I like to serve this soup with a spoonful of plant-based sour cream or plain unsweetened vegan yogurt mixed with a little lemon juice. I love the fresh, tangy contrast against the rich, savory flavor of the soup, but this is completely optional.

You can also toast a few slices of bread, add some grated vegan cheese on top, and finish them under the grill to keep that classic French onion soup feeling. Or simply serve the soup as it is. Be creative and make it your own.

Variations and Substitutions

This French-style caramelized onion and lentil soup is my own version, and it works beautifully as a base that you can adapt to your taste. Here are a few substitutions and variations that I have personally tested and that work very well.

Can I Make It Without Tomato Passata?

Yes, the tomato passata is my own addition, although tomato is very common in Mediterranean soups. You can absolutely leave it out, and the onion and lentil soup will still be delicious.

Can I Use Different Lentils?

Yes, but with green or brown lentils you will get a more rustic, less creamy soup, and the cooking time will also be different. I have also tested the same soup with chickpeas, and it turned out really well. I used pre-cooked chickpeas, so the cooking time stayed essentially the same.

Can I Make It Without Vegan Butter?

Yes, the vegan butter is not essential, although it definitely adds more flavor. You can caramelize the onions using olive oil instead, either extra virgin olive oil or a neutral oil.