Lentil Soup with butter

1 lb potato , peeled and cut into small cubes 10 ounces green lentils (not Puy) 6 garlic cloves , unskinned 1 large carrot , scraped and chopped 2 celery ribs[...]

1 1/4 cups lentils (green or brown) 2 small carrots or 2 medium carrots , diced 2 cups mushrooms , sliced 3 stalks celery , diced 1 medium onion , diced 2[...]