In a very large saute pan, heat the olive oil over high heat until very hot. Add the garlic and onion and lightly saute until fragrant, about 1 minute. Add the clams and mussels and cook just until they start to open, about 3 minutes. Add the sherry and reduce by half over high heat. Add the shrimp, whitefish, and cream and cook until the cream starts to thicken and the shrimp just turn pink, 2 to 3 minutes. Add the zest, lemon juice, 1 tablespoon of the basil, and half of the Parmesan cheese and bring to a boil. Add the cooked pasta and bring to a boil again. Season to taste with salt and pepper. Transfer to a serving platter. Sprinkle the remaining Parmesan cheese and the basil over the top. Garnish with lemon slices and serve hot. Per serving: 327 Calories (kcal); 26g Total Fat; (75% calories from fat); 9g Protein; 10g Carbohydrate; 97mg Cholesterol; 190mg Sodium Food Exchanges: 0 Grain(Starch); 1 Lean Meat; 1/2 Vegetable; 0 Fruit; 5 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.