Position rack in center of oven and preheat the oven to 350 degrees. Using 1/2 tablespoon of the butter, grease a shallow baking dish and set aside.
Place crabmeat in a large bowl and season with 1 teaspoon of Old Bay Seasoning. Cover with plastic wrap and refrigerate until ready to use.
In a medium skillet melt the remaining 2 1/2 tablespoons of the butter over medium-high heat. Add the onion, celery, and bell pepper and cook, stirring until softened, about 4 minutes. Add 1 tablespoon of the parsley and the garlic, and cook for about 2 minutes. Remove from heat and let cool.
Add the cooled vegetables to the crabmeat and toss gently to combine. Add the mayonnaise, beaten egg, lemon juice, Worcestershire sauce, and hot sauce and stir gently with a large wooden spoon. Add 1/2 cup of the cracker crumbles, 1/4 teaspoon salt, and pepper, and stir gently, being careful not to break up the crabmeat.
Season each fillet with 1/2 teaspoon of Old Bay Seasoning and 1/4 teaspoon salt and place the fillets skin side up on a work surface. Divide the crabmeat filling evenly among the centers of the fish fillets and, using your hands, roll the fillets to encase the filling. Secure with a toothpick, if necessary. Place the fillets, seam side down, in the prepared dish and top with lemon slices. Sprinkle the remaining 1/4 cup of the cracker crumbs evenly over the top of the fillets. Drizzle with the melted butter. Using oven mitts or pot holders, place the baking dish in the oven and bake until lightly golden and cooked through, about 20 minutes.