Line a large rimmed baking sheet with parchment paper or a silicone baking mat.
To make the rub, combine dijon or spicy brown mustard (2 tbsp), oil (1 tbsp), maple syrup (1 tbsp), minced garlic (4 cloves), fresh choppped rosemary (1 tbsp) or dried rosemary (1 tsp), fresh thyme leaves (2 tsp) or dried thyme leaves (3/4 tsp), salt (1/2 tsp), and pepper (1/2 tsp) in a small bowl.
Place the pork loin (1 lb) in the center of the baking sheet and rub with the mustard mixture, coating all sides.
Cut Brussels sprouts (1 lb) in half.
Cut medium apples (2 ) (suggestion: Honeycrisp, Braeburn, or Pink Lady) into 3/4 inch chunks.
Cut small red onion (1 ) into 1/2 inch thick slices.
Add Brussels sprouts, apples, and onions to the baking sheet around the pork loin. Drizzle vegetables with oil (1 tbsp), sprinkle with salt (some) and pepper (some) and toss well to coat.
Place the baking sheet in the oven and bake, uncovered, for 25-30 minutes, tossing vegetables halfway through cooking time. Pork loin is done when an instant-read thermometer reads 140℉ when inserted into the thickest part.
When pork is done, remove the baking sheet from the oven, tent loosely with foil and allow to rest for 5 minutes before slicing pork and serving with roasted veggies.