Preheat oven to 450 degrees.
Arrange the Brussels sprouts and onions around the outside of a large sheet pan. Drizzle with half of the avocado oil and toss to coat.
Place the pork tenderloin in the center of the pan. Drizzle with the remaining oil and brush to coat the pork on all sides. Season generously with salt & pepper.
Bake in preheated oven for 20-25 minutes, or until the pork registers 140 degrees on a meat thermometer.
Remove from oven and allow the pork to rest for 5 min before slicing. Return sliced pork to pan and drizzle with the balsamic vinegar and serve.