Peel the onions. Dice 1 onion finely, cut 2 into rings. Knead mince, diced onion, mustard, breadcrumbs, milk and egg. Season with 1 tsp salt and pepper. Peel sweet potatoes and cut or slice them thinly.
Form 4 large meatballs from the mince mixture. Heat 1 tablespoon of oil in a large pan and fry the meatballs for 8-10 minutes, turning them over. Heat 1 l oil in a deep fryer or pot to 175 °C.
Fry the potatoes in it in portions. Turn the potato slices while doing so. Lift out with a skimmer and let it drip off on kitchen paper. Sprinkle with salt and keep warm. Wash the salad leaves and let them drip off.
Cut the burger buns in half horizontally. Roast the bun halves, cut side down, in a pan without fat for 1-2 minutes until brown, remove. Put bacon into the pan, leave out crisp, take out.
Spread the lower half of the rolls with 1 tbsp. apple sauce each, cover with salad leaves, 1 burger, onion rings and bacon each. Add 1/2 tbsp. apple sauce to each roll. Place the upper halves of the rolls on top, press on a little and serve with sweet potato chips.