Place your 12" deep skillet into the oven and preheat it to 450°F. Once it is to temperature, let the pan continue to preheat for 10 minutes.
While the pan preheats, pat dry the pork tenderloin with paper towels then rub it with salt and pepper. Pour one tablespoon of the mustard onto each side of the pork tenderloin and use the back of a spoon to spread it around until it is completely covered.
Carefully remove the skillet from the oven and add in neutral cooking oil. Swirl it around and then use tongs to carefully transfer the pork tenderloin. Return the pan to the oven and cook for 10 minutes.
Carefully remove the pan from the oven and reduce the heat to 400°F. Flip the pork tenderloin and return the pan to the oven to cook for another 12 minutes, or until the internal temperature reaches 140-145°F. Transfer the pork to a cutting board or plate and cover it with foil.
Move the pan to the stovetop and turn the heat to medium high. Add a teaspoon of neutral cooking oil to the pan and pour in the Brussels sprouts. Cover it with the lid and let them cook for 5 minutes, only stirring once or twice. Add in the diced apples and sliced onions and cook for another 5 minutes. Do not put the lid back on.
Stir together the white wine, chicken stock and whole grain mustard. Turn the heat off and pour over the vegetables and apples. Stir.
Carefully carve the roasted pork tenderloin into slices, serve with the Brussels sprouts and apples and drizzle with leftover sauce from the pan.
For an extra flavor boost, pour any juices that released onto the cutting board or plate back into the pan before serving.If your Brussels sprouts are large, you can maintain the same cooking time by quartering them instead of halves.