Spray a 9-by-13-inch baking dish with nonstick cooking spray. Peel and remove back vein from shrimp, leaving tail intact. To butterfly, make a deep slit along inside curve of shrimp, cutting almost through to back. Arrange shrimp, slit-side up, in baking dish with tails propped up against sides of dish.
In a medium skillet, melt 2 tablespoons butter over medium heat. Add onion, green pepper and celery. Sauté about 2 minutes or until tender. Remove from heat; cool slightly. Stir in bread crumbs, crabmeat, Cajun seasoning and mustard until well mixed. Spoon 1 tablespoon bread crumb mixture on each butterflied shrimp. Melt remaining 2 tablespoons butter; drizzle over shrimp. Bake for 20 to 25 minutes or until shrimp are opaque and breadcrumbs are beginning to brown. If desired, garnish with parsley and serve with lemon wedges.