Bayou la Batter Shrimp Mornay Serving Size : 4 * 1 1/4 lb Unpeeled Large Fresh Shrimp * 1/2 lb Sea Scallops * 1/2 c Chablis or Other Dry White - Wine * 2 cn Standard Oysters, Drained - (10-Ounce) * 1/2 c Dry white vermouth * 1 tb Finely Chopped Onion * 1/4 c Plus 2 Tbs. butter Or - margarine, Melted * 1/4 c All Purpose flour * 2 c milk * 2/3 c Grated Parmesan cheese * 1/2 c Shredded Gruyere Cheese * 1/2 c Shredded Swiss cheese (2 Oz) * 1/4 ts salt * 1/4 ts pepper * 1/8 ts Ground nutmeg * 1/2 c Crushed Round Buttery - Crackers Chopped Fresh parsley