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 the butter in a frypan over medium heat. Saute the onion and celery in the butter until just tender.
Place the stuffing mix into a bowl then stir the onion and celery into it along with the butter in the frypan. Mix in the crab and prawns with their juices adding a little more liquid to get a good consistency.
Melt the remaining butter and brush some of it onto a baking tray. Stuff the stuffing mixture into the pockets of the flounder and place the stuffed fish on the baking tray.
Brush the outside of the fish with melted butter and season with the chicken and Cajun seasonings.
Bake for about 25 to 30 minutes in the preheated oven or just until the flounder flakes easily with a fork.