Place potatoes in a large saucepan; add water to cover. Bring to a boil; cover, reduce heat, and cook 15 minutes or until tender.
Place a large nonstick skillet over medium- high heat. Coat fish with cooking spray, and cook 2 minutes on each side or until done; set aside, and keep warm.
Drain potatoes, and place in a large bowl; add sour cream and next 5 ingredients, and beat at medium speed of a mixer until smooth. Set aside, and keep warm.
Preheat oven to 450°.
Heat 2 teaspoons butter in a large nonstick skillet over medium-high heat. Add carrot, celery, and leek; cook, stirring constantly, 7 minutes. Add garlic; cook 2 minutes. Sprinkle with flour; cook 2 minutes, stirring until well blended. Add clam juice and next 6 ingredients; bring to a boil. Reduce heat, and simmer until thickened, stirring constantly.
Spoon mixture into an 11- x 7-inch baking dish coated with cooking spray; top with fish. Spread potato topping over fish and vegetable mixture; coat potato topping with cooking spray, and sprinkle with paprika.
Bake at 450° for 18 minutes or until potato topping is lightly browned. YIELD: 4 servings.