2 Gala apples, cored and sliced into 10 to 12 wedges
1/2 cup dry hard cider or dry white wine
1/2 cup chicken stock
Preheat oven to 425ºF and set a large cast iron skillet over medium/medium-high heat.
Pat the pork tenderloin dry with paper towel and coat each with vegetable oil. Season liberally with salt and pepper and sear the pork tenderloins until evenly browned all over. Transfer the pork to a plate and add the butter to the pan. Add in the onions, season with a pinch of salt, and allow them to cook for about five minutes, stirring occasionally.
Meanwhile, in a small bowl, mix together the Dijon, grainy Dijon, thyme, sage, and cinnamon and brush this mixture all over the pork tenderloins.
Add the apples to the onions and continue to cook for about five to seven minutes just to give them a little colour. Carefully deglaze the pan with the cider or white wine and nestle in the pork.
Roast the pork for 15 to 17 minutes or until the internal temperature reaches 145ºF.
When cooked, transfer the pork to a cutting board, cover it with foil, and allow it to rest for five to eight minutes. Place the onions and apples back onto medium heat and add in the chicken stock. Using a wooden spoon, scrape up any browned bits off of the bottom of the pan and allow it to bubble away until the pork has rested.