Prepare biscuit mix according to package directions for rolled biscuits.
Roll to 12x7-inch rectangle.
Place on ungreased baking sheet.
Cut corned beef in 8 slices; arrange in two rows on dough.
Mix mayonnaise and mustard; spread over meat.
Sprinkle with olives, reserving a few for trim.
Bake at 425° for 15 minutes.
Top with tomato slices; dash with pepper.
Arrange cheese half-slices atop.
Return to oven till cheese melts, about 2 minutes.
Top with olive slices.
Cut in 8 squares.