**Prepare shrimp stock by boiling down 1 cup of shrimp shells in 4 cups of water. Boil shells for 30 minutes.**.
Melt butter in a saucepan; whisk in flour and chopped onion. Cook over medium heat until onion is tender. Slowly stir in stock and wine; whisk until smooth. Add cayenne and salt; Simmer 10 minutes. Add cream; simmer 5 minutes. Remove from heat and set aside.
Season shrimp and oysters with blackened seasonings (available in most supermarkets). Melt butter in hot saute pan, and saute shrimp and oysters, about 2 minutes per side. Add spinach, mushrooms, crab and green onion. Saute until mushrooms and spinach soften. Fold in sauce and bring to a simmer. Pour into heatproof dish; top with grated cheese. Place under a broiler until cheese melts. Use garlic bread as dippers.