I'm sorry, but I don't remember where I found this recipe:
1/2 c butter
1/2 c flour
1/2 c whole milk
1/2 c dry white wine
1/2 tsp black pepper
1/8 tsp cayenne pepper
1 lb shrimp
1 lb crabmeat
1 1/2 c sharp cheddar cheese
Juice the lemon. Pick crab over for bits of shell. Peel and devein shrimp. Grate cheese.
Melt butter over very low heat in heavy saucepan. Stir in flour with a wooden spoon. Cook about 1 min, then slowly start adding milk. Switch to whisk and stir until sauce is smooth, about 2 min. Add wine, lemon juice, salt, pepper, and cayenne, then whisk until sauce has a mayonnaise consistency. Remove from heat.
Preheat oven to 350. Bring 2 c water and 1/2 tsp salt to boil and add shrimp. Bring water back to boil, then let shrimp cook 1 min longer. Drain immediately.
Put crab in large mixing bowl. Chop shrimp coarsely and mix with crab, tossing gently. Add sauce and combine without breaking up crab.
Place mixture in 11x7 casserole dish, spreading it out evenly. Cover completely with grated cheese. Bake 25-30 minutes, until bubbly.