Place vegetable bouillon cubes in large heatproof bowl. Add the 1 cup boiling water. Stir to completely dissolve the bouillon cubes.
Heat vegetable oil in Dutch oven over low heat. Add leeks; cover and cook, stirring occasionally, until tender, about 20 minutes.
Increase heat to medium. Add vegetable bouillon liquid, the 5 cups water and the peas to Dutch oven; cook 10 minutes or until heated through. Add romaine. Bring to boiling. Remove from heat. Let soup stand for 5 minutes.
Working in batches, puree soup in food processor or blender. Return soup to Dutch oven. Stir in heavy cream, salt, pepper, nutmeg and potato flakes. Gently cook until heated through and slightly thickened, about 5 minutes.
To serve, ladle into soup bowls. Garnish with sour cream.