Combine mayonnaise, tomatoes, garlic, horseradish, ketchup, and cayenne in a bowl.
Toss onions, peppers, and pepper flakes together in another bowl. Drizzle with some of the pickling juice from the peppers.
Cut loaves in half and then cut in half again. Spread butter on outside if frying. Spread mayonnaise on inside. Sprinkle cheese, arrange beef on the cheese, then top with pepper salad. Place the other slices of bread on the peppers. Cook in pan or panini maker until browned and cheese is melted. Cut in half to make 14 sandwiches. (Cuisine At Home)