Trim pork loin of fat, and lay in a shallow roasting pan (metal or ceramic). Pat dry pork loin using paper towels, then sprinkle with salt and pepper.
Roast pork loin in the oven for 30 minutes to get beautiful color on your roast.
Meanwhile, Make Cherry Glaze:
Mix chopped onion, balsamic vinegar, cherry preserves, allspice, olive oil, sea salt, pepper, cayenne pepper, and frozen cherries in a medium bowl. Set aside. (This can be done ahead and kept in fridge).
After 30 minutes, pull pork loin from oven, and pour cherry glaze on the roast and around the pan, and finish roasting about 30 minutes more. (Cooking time is 60 minutes total).
Transfer pork to platter. Let rest 10 minutes. Carve and serve warm with cherry pan sauce.
Optional: pour cherry sauce from sheet pan in a small saucepan and heat on low to thicken the cherry balsamic sauce.