Preheat your oven to 375°F. Pat your pork loin dry and season with salt and pepper generously on both sides.
Heat olive oil in a large skillet over medium-high heat. Brown the pork loin on both sides (about 2–3 minutes per side) and transfer to a roasting pan.
Add the apples and onions to the pan and cook about 5 minutes until slightly browned, scraping up the browned bits from the bottom of the pan. Arrange apple and onion slices around the pork.
In a bowl, whisk together apple cider, apple cider vinegar, garlic, Dijon mustard, brown sugar, thyme, sage, 1/2 teaspoon salt and black pepper to taste.
Pour the cider mixture over the top of the pork.
Cover the pan with foil and bake for 45 minutes. Remove foil, baste with pan juices, and continue roasting uncovered for 45 minutes more, or until the pork reaches an internal temperature of 145°F. (A digital thermometer is perfect for this!)
Let the pork rest for 10–15 minutes before slicing.
Strain the juices into a medium saucepan. Heat over medium high heat. Add 2 tbsp butter. Whisk together the cornstarch and water to make a slurry and pour into the gravy. Whisk and simmer for about 5 minutes, until slightly thickened. Spoon over sliced pork.
