In a large skillet melt the 3 tablespoons butter over medium heat. Cook and stir leeks in hot butter for 2 to 3 minutes or until tender. Add the flour to the skillet; cook and stir about 1 minute. Stir in the milk; cook and stir until mixture begins to boil. Stir in the crabmeat, peas, dry sherry (if desired), Cajun seasoning, and, if desired, the ground red pepper. Spoon the crab mixture into a 2-quart square baking dish.
In a small bowl stir together the 2 tablespoons melted butter and, if desired, the Dijon-style mustard. Brush one sheet of phyllo dough with butter mixture; fold in half crosswise. Repeat with other sheet. Stack the two sheets. Using a sharp knife, trim phyllo to about a 9-inch square. Place phyllo stack over crab mixture; tuck edges under. Brush any remaining butter mixture over top. Bake in a 375 degree F oven for 25 to 30 minutes or until heated through. Serve immediately.