Heat 3 tablespoons butter in saucepan over medium heat. Cook 1 small onion, chopped and 2 leeks (white parts only) chopped fine, until translucent, 5 to 7 minutes. Add 1 large all purpose potato, peeled and diced, and 2 1/2 cups chicken broth. Simmer, covered, until potato is very tender (20 to 25 minutes). Puree soup in blender, strain into saucepan. Add 1/4 to 1/2 cup milk and season with 1/4 teaspoon each salt and white pepper. Heat through. Ladle into serving bowls. Top with fried onion rings. Makes 2 servings.