Preheat oven to 400° F.
Place fillet rounded side up on cutting board. Form a pocket by making a slit lengthwise on thicker part of fillet, taking care not to cut all the way through.
Flake crab meat, and check for pieces of shells.
In a large skillet, melt butter over medium heat. Sauté onion and celery until tender. Add garlic; cook 2 more minutes. Add parsley and green onions, and remove from heat.
Stir in lemon juice, cayenne pepper and breadcrumbs.
Add crabmeat and prawns or shrimp, tossing gently.
Stuff fillet pockets with seafood mixture. Sprinkle with Parmesan cheese, paprika and melted butter.
Bake 20 to 25 minutes or until done.
Garnish with lemon slices and parsley
Serves 4