1Combine wine, onion, salt and scallops in large saucepan. Bring to a gentle bubbling (not boiling) point. Reduce heat and simmer, just till scallops are done. Remove scallops from pan, cut in half if large pieces.
2To the liquid in pan, add 2 Tablespoons of butter, lemon juice and liquid from mushrooms. Heat till butter melts. Add enough water to make this 2 cups.
3In another saucepan, melt remaining butter. Add flour. Stir a few minutes over moderate heat. Gradually add wine mixture and half and half. Cookk, stiring, over moderate heat till thickened. Add cheese, stirring until cheese melts. Add scallops, mushrrom, shrimp and crab. Heat through. Serve in casserole.