Preheat oven to 375ºF.

Sauté celery, onions, and garlic in ¼ cup butter over low heat. Add bread cubes, shrimp, crabmeat, parsley, and egg. Mix well. Season to taste with salt, black and cayenne pepper. Set aside. Split thick side of flounder lengthwise and crosswise forming a cross; loosen meat from bone to form pocket for stuffing. Melt ¼ cup butter; brush fish well with melted butter, salt, and pepper.

Fill pockets with seafood stuffing mixture. Melt remaining ¼ cup butter in shallow baking pan. Place fish in pan, making sure not to overlap. Cover and bake for 25 minutes, until fish flakes easily with a fork. Remove cover and bake another 5 minutes. Serve at once.