Peel and cut the potato and carrot into cubes.

Sort and rinse the lentils well. Place them in a pot with the water, then add the onion, potato, garlic, and carrot. Bring to a boil, then reduce the heat.

Once the vegetables are tender, turn off the heat and blend the mixture with an electric blender until smooth.

Return the soup to the heat. If it's too thick, add a little water. Stir in the melted butter, salt, and black pepper. After it comes to a boil, remove from heat.