Heat the butter in a large skillet or nonstick sauté pan over low heat. Slather 4 slices of bread with the mustard, then divide the cheese, apples, and bacon among them. Top with the other bread slices and add to the hot pan. The key to a great grilled cheese (i.e., crispy crust, fully melted cheese) is patience, so cook these sandwiches slowly until each side is deep brown and crunchy, about 10 to 12 minutes total.