1 cup extra virgin olive oil
1/2 cup balsamic vinegar
4 tablespoons cider vinegar
6 garlic cloves finely chopped
2 tablespoons, chopped fresh rosemary, plus rosemary branches for garnish
4 tablespoons honey
2 tablespoons Dijon mustard
coarse salt and freshly ground pepper
2 1 pound pork tenderloins
1 red onion cut into thick slices
Make the glaze: In a medium bowl, combine olive oil, both vinegars, garlic, rosemary, honey, mustard, salt and pepper to taste. Whisk to combine.
Place the pork tenderloins in a shallow dish or plastic bag. Pour half of the glaze over the pork and refrigerate overnight.
Preheat oven to 375 degrees.
Remove the pork loins from the marinade. Cover the bottom of an oven-proof skillet with the onion slices, and place the tenderloins on them. Roast until a meat thermometer inserted into the center registers 150 degrees, about 25 minutes.
Remove pork tenderloins and onions from skillet and cover with foil so the juices redistribute. Place skillet on burner set to high heat. Add remaining unused marinade to the pork drippings in the skillet and deglaze the pan. Continue to reduce the glaze until thick and syrupy.
Slice the tenderloins into 1/2 in thick slices and drizzle with the reduced marinade.