Make a generous horizontal slice in the side of each chop so there is room for a substantial amount of stuffing. Salt and pepper both sides of chop, rub with olive oil and set aside to make the stuffing.
In a large heavy skillet over medium heat, add 1-2 T of olive oil with the sliced onion. Sauté for 5-8 minutes, until onion slices are soft and caramelized. Add garlic, thyme, apple slices and a pinch of salt and pepper. Continue cooking for 3-5 minutes until apples soften. Add 1 T apple cider vinegar to deglaze pan and remove from heat. Add 1 T maple syrup and walnuts. Place mixture in bowl, set aside to cool and remove thyme sprigs.
Preheat oven to 400 degrees. To stuff pork chops, use about 1/4 – 1/3 cup of filling per chop. Reserve remaining filling for sauce. In a clean heavy skillet over medium high heat, add 1-2 T of olive oil. When hot, add chops to pan and sear each side well, 3-5 minutes per side. Transfer skillet to oven and cook for 5 minutes. Check meat thermometer for internal temperature —145 degrees. Continue to cook if necessary. Remove from oven, transfer chops to a platter and cover with foil
In same pan over medium heat, deglaze pan with remaining 2 T apple cider vinegar. Add chicken stock, Dijon mustard, and any remaining filling. Cook for 3-4 minutes. Add butter and the remaining 1-2 T maple syrup (depending on desired sweetness). Continue to cook and stir until butter is melted. Add salt and pepper to taste. Plate pork chops and drizzle sauce over the top. Serves 4.