Red lentil soup

Another one of my mother's recipes. Super easy and tasty. My 3 year old calls it 'Magic Soup' because I told him he would grow if he ate it.

1 cup of red lentils
4 cups of water
1 onion chopped
1 sweet potato chopped
salt & pepper to taste
2 tbsp butter (optional)

Rinse lentils well. Bring water to a boil, add lentils, onion, sweet potato, and salt & pepper. Boil for 15-20 mins. Remove from heat, let cool for 5 mins. Blend in food processor or blender, add butter. Serve and enjoy.