Preheat the oven to 300 degrees F (150 degrees C). Lay the fish down dark side up. Make an incision next to the backbone on both sides, stopping before the head and tail. Slip the knife in between the flesh and backbone and run the knife down the ribs on both sides of the backbone.
Melt 1/2 cup of butter in a skillet over medium heat. Saute the onion and celery in the butter until just tender. Place the stuffing mix into a bowl, and stir the onion and celery into it along with the butter in the pan. Mix in the crab and shrimp with their juices, adding a little more liquid, or more stuffing to get a good consistency.
Melt the remaining butter, and brush some of it onto a cookie sheet. Stuff the stuffing mixture into the pockets of the flounder, and place the stuffed fish on the cookie sheet. Brush the outside of the fish with melted butter, and season with the Old Bay and if desired, Cajun seasoning.
Bake for about 25 to 30 minutes in the preheated oven, or just until the flounder flakes easily with a fork.