In a bowl, mix 1/2 tbsp olive oil, dijon mustard, salt and pepper together and rub over the tenderloin. Let marinate 30 minutes to 2 hours in fridge, once done marinating, remove from fridge and let rest for 10-15 minutes.
While the pork is marinating, slice your onion, chop your apple, dice your rosemary and mince your garlic (if you haven’t done so already).
Preheat oven to 400°. Heat 1 tablespoon oil in large cast iron or oven-safe skillet over medium-high heat. Wipe excess marinade from pork and cook until browned on all sides, about 2-3 minutes on each side.
Put your pork in the oven and cook 5 minutes.
While pork cooks, mix together whole grain mustard and maple syrup and set aside
Remove pork from the oven and add your apple, garlic, and onion around the edges of the pork. Lightly salt and pepper the apple, garlic, and onion, give them a quick toss and place back in the oven for another 15-20 minutes, or until pork reaches 145 degrees.
Remove from the oven and let the pork rest. If your apple or onion aren’t done to your liking, sear a little longer and then mix in the sauce with the apple onion mix or pour over the pork (or use it to dip!). Slice up the pork and serve with apple and onion mix with your sauce.