Preheat oven to 450 degrees. Season pork on all sides with salt and pepper.

Heat 2 tablespoons oil in a large nonstick ovenproof skillet over medium-high heat. Add pork and sear until all sides are brown, turning occasionally, about 5 minutes. Transfer pork to plate to cool slightly. Spread Dijon mustard over top and sides of pork and sprinkle with half of the thyme. Add remaining 1 tablespoon oil to the skillet. Add onion slices and apples, saute over medium heat until golden for about 5 minutes. Spread evenly in skillet and sprinkle with salt and pepper. Place pork atop the apple-onion mixture.

Transfer the skillet to oven and roast until apple-onion mixture is soft and brown and meat thermometer inserted into center of pork registers 150°F, about 15 minutes. Transfer pork to platter and tent with foil. Let stand 5-10 minutes.

While the pork rests, pour apple cider and chicken stock over apple-onion mixture in skillet, lifting any brown bits from the bottom. Sprinkle apples and onions with additional thyme. Bring to a simmer and cook until reduced by half. Add butter and stir until melted. Cut pork on diagonal into 1/2-inch-thick slices. Spoon apple-onion mixture onto plates. Top with pork and serve.