Preheat the oven to 400° F.
Heat a non-stick or cast-iron skillet over medium-high heat. Add the pork tenderloin to the skillet and sear for 1-2 minutes on each side until brown crust forms on the meat.
Transfer the pork tenderloin to a sheet pan. Place the Brussels sprouts, chopped sweet potatoes, apples, and red onion on the sheet pan around the pork. Toss the veggies with olive oil and season with salt and pepper.
Roast the pork and veggies in the oven for 25 to 30 minutes or until the pork registers 150° with a thermometer in the thickest part of the tenderloin. Flip the veggies once during roasting.
While the pork is roasting, make the apple cider gravy. Heat the skillet used to sear the pork over medium heat and add 1 tablespoon butter. Cook the minced garlic in the butter until fragrant, about 1 - 2 minutes.
Add the fresh thyme, apple cider, and Dijon mustard. Stir until the mustard is smooth and incorporated. Add the heavy cream and stir until rich and creamy. Remove from the heat as it can reduce further if left on the heat too long.
Remove the pork from the oven and tent with foil before slicing for about 5 minutes.
Plate the veggies and pork on a platter and serve with the apple cider gravy.