Thaw the Flounder fillets, drain, pat dry.
Preheat the oven at 400 degrees F.
Using a bowl, combine the crab meat, green pepper, mustard powder, Worcestershire sauce, salt, white pepper, and the 3 crushed saltine crackers. Add the egg white and 1 tablespoon of mayonnaise and stir.
Brush the Flounder fillets with the melted butter and place them in a lighted greased baking dish.
Spoon the crab mixture over the fillets and drizzle with any remaining butter.
Bake them for 15 minutes at 400 degrees.
Using a small bowl, lightly beat the egg yolk; stir in 5 Tbsp of mayonnaise.
Remove the Flounder fillets from the oven and spread this mixture over the stuffing. Sprinkle paprika and parsley on top and return it to the oven.
Increase the oven temperature to 450 degrees and bake for about 6 minutes, until golden and bubbly.
Serve it hot.