2 medium shallots, chopped
1 teaspoon olive oil
1 large Idaho potato, peeled and cut in cubes
1 teaspoon salt
3-4 cups water
1pound frozen peas
1 tablespoon sour cream, mixed with 1 teaspoon water
Cook shallots in oil in a 3-quart heavy saucepan over medium heat, stirring, until softened, about 2 minutes. Add potato and salt and cook, stirring, 2 minutes. Add water and simmer, covered, until potato is tender, about 15 minutes. Add peas and simmer, uncovered, 2 minutes.
Using a hand blender or food processor, purée soup then force it through a fine strainer back into the saucepan. Reheat soup and season with salt and pepper.
Drizzle each portion with sour cream.