For the catfish: To marinate the catfish, place the fillets and buttermilk in a flat bottomed container. Allow to marinate in the fridge for a minimum 30 minutes, maximum 1 day.
Preheat a fryer to 350 degrees F with vegetable oil. Before frying, remove the catfish from the marinade and allow any excess buttermilk to drip off. Season the cornmeal with salt and pepper. Coat a fillet with flour, then egg wash, then seasoned cornmeal and repeat the process with the remaining fillets before frying. Fry until the catfish reaches 145 degrees F, 4 to 5 minutes. Remove from the fryer and allow to drain any excess oil on a plate with paper towels.
For the sauce: In a medium saucepan over medium heat, add the butter and melt. Next, add the onions, peppers and celery and sweat until translucent. Then stir in the flour, seafood seasoning and garlic to thicken the butter. This will take 1 to 2 minutes.
Next, add the cream and reduce the heat to medium-low. Cook until the cream sets. Finally, add the crawfish and whisk to blend, cooking for final 2 minutes.
To serve, place 1/2 cup grits or polenta on the plate, then a quarter of the prepared crawfish sauce and top with a crisp cooked fish. Finish with green onions or chives.