Make mustard mayonnaise:
In a bowl, whish together 2/3 c. mayonnaise, sweet pickle, and Dijon mustard and reserve.
Make crab po' boys:
In a small heavy skillet, cook onion and bell pepper in butter over moderately low heat, stirring occasionally, until softened. in a bowl, stir together onion mixture, crab meat, scallion greens, 6 Tbsp. mayonnaise, egg, 1 Tbsp. bread crumbs, Worcestershire sauce, cayenne, and salt to taste. Put remaining 1/2 c. bread crumbs in a small bowl and put a piece of plastic wrap on a work surface. Form one fourth crab mixture into a 3-inch patty (it will be soft) and coat with bread crumbs, transferring to plastic wrap. Make 3 more patties in same manner with remaining crab mixture and bread crumbs. Refrigerate several hours. In a 10-inch heavy deep skillet, heat 1/4 inch vegetable oil over moderately high heat until hot but not smoking and fry patties until golden and cooked through, about 2-1/2 minutes on each side, transferring to paper towels to drain. Spread bread with reserved mustard mayonnaise and sandwich crab and lettuce between bread.
» Serves 4.