Dice the onions. Peel the apple and grate 3/4 of it with the large grater. Cut the rest into 0.5 cm cubes.
Cut the pork fillet into 3-4 cm thick medallions and fry them with a little oil. Season the medallions with salt and pepper, remove them from the pan (a little earlier than normal as they will continue to cook in the oven) and keep them warm in the oven at 80 ° C.
Sauté the onions with the grated apple in the roasting tray of the medallions. Deglaze with balsamic vinegar. Add the cream and process everything a little with the hand blender. Add the apple cubes and simmer gently for about 5 minutes.
Spaetzle go well with the dish.
Instead of balsamic vinegar, you can deglaze the sauce with cognac and get a slightly spicier, less acidic sauce.