Preheat the oven to 180C fan/gas mark 6. Line a baking tray with non-stick baking parchment, then arrange the slices of pork on the tray. Sprinkle the pork with salt and pepper and scatter over 10 thyme sprigs.
Roast until crisp and golden brown, about 15-20 minutes. Take the tray out occasionally and carefully pour the fat off into a bowl – save this for later.
Sauté the apple and onion wedges along with the rest of the thyme and lots of black pepper in 2-3 tablespoons of the reserved pork fat (about 5 minutes – you want the apples to be al dente).
Add the slices of roast pork to the cooked apples and onions and season to taste with salt and pepper. Transfer to a serving dish and serve with rye bread (you could also use crispbread or sourdough).