In a large pan, stir-fry leek, mushrooms and garlic for 3 to 4 minutes until leeks are soft. Stir in broth, water, lentils, sweet potato, and bay leaf. Bring to boil then simmer uncovered until lentils and sweet potatoes are soft, about 30 to 40 minutes. Remove bay leaf and puree 2 cups of soup until smooth or use an immersion blender; return to pot, stir in basil and pepper to taste. Just before serving stir in as much as you dare fresh spinach. It will melt in the pot. If you prefer fill individual soup bowls with spinach and spoon hot soup over the spinach. Top with a sprinkle of more fresh basil.