Cut trimmed tenderloin into 2" thick slices, then quarter each slice and place in a resealable plastic bag.
Combine 3 tbsp white wine vinegar, 5 tbsp olive oil, 6 cloves grated garlic, 1 tsp Dijon mustard, 2 tsp dried oregano flakes, 1 sprig finely chopped fresh rosemary, 1 tsp sweet Hungarian paprika, 3/4 tsp fresh ground black pepper and 1 1/2 tsp kosher salt in a small bowl. Mix to combine.
Add about 1/3 of the marinade to the cubed pork tenderloin and message to make sure that the marinade covers all sides of the pork. Refrigerate pork for 30 minutes and up to 2 hours. Reserve the rest of the marinade to use as a dressing.
Remove pork from the refrigerator at least 1/2 hour before cooking.
Remove pork from marinade, pat dry with a paper towel and season with kosher salt and freshly ground black pepper. Discard the remaining marinade from the bag.
Heat a medium sized skillet over medium high heat for about 2-3 minutes. Add a drizzle of olive oil and when the oil starts to shimmer, add the drained marinated pork and 1/2 medium red onion quartered and separated into leaves. Don't crowd the pork, make two batches if it is required.
Sauté the onions and pork. Cook pork and onions undisturbed on one side for 3 minutes, then turn to the next side and sauté for another 2-3 minutes, or until both the onions and the pork are slightly charred.