Directions: Prepare the sauce; saute the onions in 2 cubes of butter in a small pan over the law heat, without browning. Add the vinegar and bring to the boil. Simmer, until the liquids are reduced to about to the level of onions. Add the cream, bring to boil and remove from the flames. Gradually stir in the rest of the butter, whisking constantly, until totaly absorbed. Season with salt and white pepper.
Heat the oven to 250 degrees C. Grease the baking tin.
Heat the oil in a frying pan until it smokes a little. Season the fillets with pepper and place into the pan, skin side doun. Salt and pepper the pink skinless part and gently press on fillets with the hand. After 1 minute, turn the fillets, fry for 1 minute longer and transfer to the baking tin. with a skins up. Bake for 4 minutes. Remove from the oven. transfer to a serving plate and pour the souce over.