Fry the vegetables. Put the frying pan on the fire and melt the butter in it. Put the onion, carrot and celery in the frying pan. Stir and fry on low heat until the onion is soft.
Prepare the lentils. Pour water and salt into the pan with the vegetables. Add the lentils. Bring the mixture to a boil, add the garlic and cover with a lid. Cook for 15-20 minutes on low heat until the lentils are ready. Stir and remove from heat.
Make a puree. Use an immersion blender to puree the lentil mixture. You should get a thick, smooth mass.