In a large saucepan over medium heat, melt 4 tablespoons of butter. Remove from heat and whisk in flour. Pour in 1/2 cup of heavy cream and whisk until smooth, return to heat.
Add cheese and stir until creamy. Remove from heat and whisk in remaining cream. Stir in wine, paprika, crabmeat, and spinach. Season with garlic, salt, rosemary, and thyme. Mix in the onion and mushrooms.
Pour into prepared casserole dish.
Top with bread crumbs. Drizzle 1 tablespoon melted butter over the top.
Bake in preheated oven for 45 minutes or until top is browned and bubbly.