On 4 large buttered squares of heavy duty aluminium foil, place equal amounts of fish. In a small saucepan, melt the margarine. Add lemon juice, parsley, dill weed, salt, and pepper. Pour equal amounts over the fish. Sprinkle with paprika, and top with sliced onions. Wrap foil securely around the fish, leaving enough space for the fish to expand. Grill 5 to 7 minutes on each side, or until the fish easily flakes with a fork. Refrigerate leftovers. Makes 4 servings.