Prepare the sauce and set it aside.
Make shrimp stock by boiling shells in water.
Boil the shells for 30 minutes.
In a saucepan, melt the butter.
Whisk in flour and chopped onion.
Cook over medium heat until the onion softens.
Slowly stir in stock and wine.
Whisk until the mixture is smooth.
Add cayenne and salt.
Simmer for 10 minutes.
Add cream and simmer for 5 minutes.
Remove the sauce from heat and set aside.
Season shrimp and oysters with blackened seasoning.
Melt butter in a hot sauté pan.
Sauté shrimp and oysters for 2 minutes on each side.
Add spinach, mushrooms, crab, and green onion.
Sauté until mushrooms and spinach soften.
Fold in the sauce and bring to a simmer.
Pour into a heatproof dish.
Top with grated cheese.
Place under the broiler until the cheese melts.
Serve with garlic bread for dipping.