Transfer the raisins to a small cup and add cold tap water until they are fully submerged. Let the raisins soak in the water for at least 30 minutes until needed later on.

Then season the pork tenderloin on all sides with some pepper and salt.

Then remove the pork tenderloin and let it rest on a clean plate for now. Pour some some extra olive oil in the same pan you cooked the pork in (don't clean it but keep the remaining pork juices to add extra flavor to the sauce) and add the chopped yellow onion.

Place the pan over over medium heat and gently cook the onion for about 5 to 7 minutes until transparant. Then pour in the chicken stock and the vinegar. Season with a pinch of pepper.

Stir the onion sauce well. Bring it to a light boil and then turn the heat low. Drain the soaked raisins and add them to the sauce.

Add the pork back to the pan as well and cover it.

Cook for another 7 to 10 minutes until the sauce starts to thicken. You can always add flour or sauce thickener. I prefer to take the pan off the heat in the end and stir in a tablespoon of cold butter.

Check the seasoning and add extra pepper, salt or vinegar to taste if necessary. Slice the pork up and place it onto warm plates. Drizzle with the onion sauce and serve hot.