Place a medium or large pan over medium heat. Add 1 tablespoon of oil and preheat until the oil is shimmering.
Once the oil is shimmering, add half a thinly sliced yellow onion and cook for 5 minutes until the onions are softened.
While the onions are cooking, add a dusting of salt and pepper to one side of the pork chop.
After the onions are softened, move them into a pile on the side of the pan and continue cooking them in a pile until the pork chops are fully cooked. Just make sure to move the onions around and rearrange the pile every so often to make sure they continue cooking without burning, but keep them off to the side so you have space to cook the pork chop.
Place the pork chop into the pan, salt and pepper side down, and cook for 4 minutes on the first side. Salt and pepper the second side while it is cooking.
Flip the pork chop and cook for another 2 to 3 minutes. Move all the onions over on top of the pork chop at this point, while the chop is still in the pan.
After 2 or 3 minutes, move the pork chop and onions to a plate while you prepare the bun.
Slice a bun and spread yellow mustard on both sides. Add the pork chop and onions to the middle and serve.