Wash the fillets, dab dry and season with pepper. Wrap 5 slices of bacon around each fillet, place in an ovenproof dish and prick several times with a fork. Peel onions and garlic, dice very finely. Mix with sugar, tomato paste, vinegar and soy sauce. Pour the marinade over the fillets and turn them over. Cover and chill overnight.
Place the fillets together with the marinade on the fat pan. Roast in the preheated oven (electric cooker: 125 °C/circulating air: 100 °C/gas: see manufacturer) for approx. 1 1⁄2 hours.
Remove the fillets and let them drip off on kitchen paper. Pour frying stock through a sieve into a pot, bring to the boil. Stir starch and 2 tablespoons of water until smooth. Thicken sauce with it, season to taste. Heat oil in a large pan. Brown the fillets in it all around. Arrange fillets and drizzle with some sauce. Serve the rest of the sauce and roasted potato salad with it.