For the rolls, peel and wash the potatoes, cover and cook in salted water for about 20 minutes. Drain. Add 2 tablespoons of milk and butter and mash to a creamy puree. Season to taste with salt and nutmeg.
Mix the mashed potatoes with sour cream, 1 egg, sugar, 2 tablespoons of oil and 1 pinch of salt. Then mix dry yeast and 200 g flour and stir in well. Knead the remaining 250 g flour with your hands for 5-8 minutes.
Cover the dough and let it rise in a bowl in a warm place for about 45 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line a baking tray with baking paper. Knead the dough again, divide into 8 pieces and form into round, even rolls.
Place on the tray and leave to rise for another 15 minutes. Mix 2 tablespoons of milk and 1 egg, spread on the rolls. Bake in a hot oven for about 20 minutes until golden brown. Take the rolls out of the oven and let them cool down.
For the burger meatballs season the mince with approx. 2 tsp salt and approx. 1 tsp pepper. Form 8 flat meatballs (as big as the rolls). Heat 2 tablespoons of oil one after the other in a coated frying pan.
Fry the meatballs in it in 2 portions on each side for about 5 minutes.
Clean, wash and drain the lettuce and divide into individual leaves. Wash and slice the tomatoes. Peel and thinly slice the onion.
Preheat the oven grill. Coarsely grate and mix both cheeses. Put the meatballs on the baking tray. Sprinkle with the cheese and bake under the grill for about 2 minutes. Take out.
Cut the buns in half. Cover the lower halves of the rolls with lettuce, tomatoes, onion, jalapeños and meatballs. Add 1 tablespoon of ketchup to each roll. Place the upper roll halves on top.