Sweet Potato Soup

Chop 1 onion, 1 clove of garlic, and 10 grams fresh ginger. Clean 1 tomato and cut in cubes. Peel 440 grams of sweet potato and cut in cubes as well. Put all together with 2 tablespoons of olive oil in a pot and stew. Add half liter of vegetable stock, salt and pepper and bring it to boil. Let it simmer for 15 minutes with a lid on the top.
Cut 1 tomato in quarters, core it and cut in cubes. Chop 2 tablespoons of parsley.
Puree the soup with a blender, add 3 tablespoons of sour cream and bring it to boil again. Season with salt, pepper and 1 to 2 teaspoons of lemon juice. Sprinkle the soup with the cut tomato and parsley and serve it.