To make shrimp stock, combine reserved shrimp shells, chopped vegetables and water in a large stockpot; simmer over medium heat for about 30 minutes until liquid is reduced to 3 cups; strain and set aside.
Sauté shrimp in 1 tablespoons butter until turning pink and set aside.
Make roux by melting 4 tablespoons butter in saucepan until about to turn brown; whisk in 4 tablespoons flour until mixture thickens to a paste; cook until it turns a light, golden brown. Set aside.
In a saucepan, melt 1 tablespoons butter and add garlic and remaining onion; sauté over medium heat 2 minutes.
Add shrimp stock and crushed bouillon cube; stir and let simmer.
While the stock is simmering, melt 1 ¼ stick of butter in a separate saucepan until it turns golden brown then set aside.
Add roux to stock mixture and stir until fully incorporated; simmer for 3-5 minutes.
Whisk melted butter into stock until fully incorporated then whisk in ¼ cup Madeira wine until fully incorporated; mix in Cajun seasoning and simmer for 1-2 minutes.
Add 6 oz. back fin crab meat and cooked shrimp into sauce; simmer for 3-5 minutes and then ready to serve.