Chop all the vegetables into medium-sized pieces. In a pot over medium heat, heat the oil. Add the onion, garlic, leek, and celery, and sauté for 10 minutes, stirring occasionally.

Add the carrot, potato, and zucchini, and stir to combine. Rinse the lentils well, drain, and add them to the vegetables. Pour in 2 cups of water (about 480 ml), cover the pot, reduce the heat, and let it simmer until the vegetables and lentils are soft and fully cooked.

Add salt and cumin. Blend the soup with an immersion blender or in a regular blender until smooth and creamy. Return the pot to the heat for 5 more minutes, adding the butter.

Serve hot with a squeeze of lemon juice and toasted bread.