Make thyme mustard butter by mashing together (in a bowl) butter, mustard and thyme leaves.
Preheat oven to 350.
Cut apple into 1-inch wedges. Slice potatoes into thin rounds. Thinly slice onion. Toss potatoes with half the apple, half the onion, salt and freshly ground pepper. Butter the bottom of an oven-proof dish that is large enough to hold the pork chops in one layer. Add the potato, apple and onion mix and dollop with the thyme mustard butter, using about half or even less if you like.
Melt about 2-3 Tbls. mustard butter in a frying pan. Season pork chops with salt and pepper and quickly brown on both sides. Place chops on top of potatoes and apples in baking dish and then put the rest of the cut apples and onion on top. Deglaze the frying pan by adding cider, letting it bubble and scraping the juices that have stuck to the bottom. Pour this over the chops and bake, uncovered for about 45 minutes, turning occasionally. Serve with a bit of the remaining thyme mustard butter on top of each helping.