Cover 1 side of outdoor grill rack with aluminum foil; prepare grill for direct grilling over medium-low heat.
Place brats on uncovered hot grill rack; place bacon on foil-covered hot grill rack.
Cover and cook brats 15 minutes or until internal temperature reaches 160°, turning frequently; cook bacon 13 minutes or until bacon is crisp, turning once.
About 3 minutes before brats are done, place buns, cut side down, on hot grill rack; cover and cook 3 minutes or until grill marks appear. Chop bacon.
In a large bowl, whisk mayonnaise, vinegar, mustard, celery seed, salt, and pepper; fold in apple, cabbage, and onion. Makes about 3 cups.