Bake the casserole, covered with its lid or with foil, for 30 minutes. Run it briefly under the broiler to brown before serving. Combine all ingredients in a blender and process until barely blended. Place in plastic bag to cool for ten minutes. Peel the chiles under cold water.
Slice open one side and remove seeds.
Sprinkle crab with lemon juice. Stir in mustard, mixing well. Add egg whites and basil and blend. Add bread crumbs and toss mixture together. Open chiles flat and fill with filling, then fold chiles together.