In medium bowl, combine apples, 2 tbsp oil, vinegar, sugar, ¾ tsp salt, and a pinch of pepper. Toss to coat, set aside.
Preheat oven to 450. Toss potatoes with 2 tbsp oil, and season with salt and pepper. Roast til tender, ~15min.
Melt 1.5 tbsp butter in skillet over medium heat. Split buns in half, and place cut sides down on skillet to toast, ~1-2min.
In same skillet, heat 1.5 tbsp each of butter and oil over medium heat. Add onions and a pinch of salt. Cook, stirring occasionally, til browned, ~7-10min. Stir in 1 tbsp butter and ⅔ of the minced garlic, cooking ~30seconds, til fragrant. Remove from heat.
Push potatoes to one side (this is a good time to toss them), and place brats on baking sheet, cut side down. Roast til sausages are warmed through, ~8-10min.
In another bowl, combine dill, mayo, ⅓ of the minced garlic, 2 tsp vinegar, and ¾ tsp sugar. Set aside.
Mix potatoes with mayo mixture, til combined.
Spread mustard on buns, and top with sausages, onions, and pickled apples. Serve with side of potato salad and more apples.