Gather the ingredients.

In a large pot, add the olive oil and sauté the onion and carrots over medium-low heat until slightly softened (around 4 minutes).

Add the tomato paste and cook for 1 minute, stirring constantly.

Add the broth, lentils, and 1 cup water.

Bring the soup to a boil, then reduce heat and partially cover the pot. Allow the soup to simmer for at least 30 minutes, or until the lentils are soft to your liking.

While the soup cooks, juice half of a lemon.

Test the lentils. If they’re still too hard, let them cook longer. Add a little water if you need to.

When I checked my lentils after 30 minutes, they were still slightly tough, so I added another cup of water and let them simmer for an additional 15 minutes.

Let the soup cool slightly, then transfer half of it to a blender. Purée until smooth.
If you have an immersion blender, this step will be easier.
If you like a smooth soup, purée the entire batch instead of just half.

Add the lemon juice to the purée in the blender, and process once more. If the mixture in the blender is too thick or won’t blend well, add a little water to loosen it.

Combine the purée with the remaining lentil mixture in the pot, and re-warm the mixture if you are serving it immediately.
Taste, and adjust the seasonings as necessary.
If the soup is too thick, add water to thin it to the desired consistency.