Boil the potatoes in salted water, mash and stir with milk. Mix in the fried onions and herbs, season with salt and keep warm.
Cut the apples and onions into small cubes. Heat the oil in a coated pan and fry both for about 5 minutes. Stir in the mustard and deglaze with apple juice and stock. Season with salt and pepper and simmer a little.
Cut the pork fillet into approx. 1 cm thick slices, season with salt and pepper and place in the sauce. With the lid closed, let it simmer for about 10-15 minutes.
Place the mashed potatoes in the middle of the plate, put the apple sauce around it and place the pork tenderloin on top of the puree.