Preheat grill or grill pan for direct grilling on medium-high. Grill 4 bratwurst sausages for 10 to 12 minutes or until cooked through, turning often. Meanwhile, into a large bowl, grate 2 large unpeeled Granny Smith apples and ½ small red onion. Toss with ⅓ cup mayonnaise, 1 tbsp cider vinegar, 1 tsp spicy brown mustard and ¼ tsp celery salt. Divide among 2 mini French loaves or 4 crusty rolls.