To a 1-gallon plastic bag add paprika, ground thyme, salt, ground black pepper, basil, and chili powder, and shake to combine. Add the crawfish and shrimp to the bag in batches and shake to coat. Add the crawfish and shrimp a little at a time to the boiling water so as not to stop the boil, and let cook until pink and the crawfish is curled. As they are cooked, use a slotted spoon to remove the crawfish and shrimp to a platter and set aside until they are cool enough to handle. Remove the shells from the crawfish and shrimp and add the shells back to the pot of cooking liquid, and allow to reduce by two-thirds. (The shells of crawfish are removed by cracking the tail with both hands and forcing it back out through the curve of the tail.) Reserve the crawfish and shrimp meat until needed. When reduced, strain the cooking liquid into a bowl.
For the sauce:
In another saucepan, combine the garlic, scallions, bay leaf, nutmeg, if using, and butter over medium heat. Add flour to make a roux and allow it to cook until blond, then begin whisking in the milk. Into the bowl of reduced seafood cooking liquid, whisk the Cheddar and Gruyere cheeses. Add this to the milk mixture and simmer for 15 minutes, stirring frequently. Remove saucepan from heat, season sauce, to taste, with white pepper, and salt if needed. Remove and discard bay leaf. Immediately put a sheet of plastic wrap on top of the sauce to prevent a skin from forming.
Preheat oven to 350 degrees F.
Fold the crawfish and shrimp into the sauce a little at a time so you can control the amount of sauce to the amount of seafood. (Do not feel compelled to use all of the sauce.) Spread the mixture into a 3 quart casserole dish. Sprinkle with the topping and heat about 5 minutes in the oven until the topping is browned. Serve family style, garnished with parsley sprigs.