Preheat your oven to 475 F/ 250 C. wipe the mushrooms with a damp towel and place them in an oven-safe skillet. Slice the brussel sprouts in half, and transfer them to the skillet.
Step 2
Mince the garlic and add it to the vegetables. Add the olive oil, ½ tsp salt, crushed red pepper, and thyme. Toss to combine.
Step 3
Rub the beef with mustard, and season with ½ tsp of salt and black pepper. Place the beef loin on top of the vegetables. Slice the butter and add it to the top of the meat.
Step 4
Transfer the skillet to the oven. Roast until the beef register 120-125 F for medium-rare, about 25-35 minutes. Take the skillet out of the oven and place it on the counter.
Step 5
Cover the skillet with aluminum foil. Let it rest for 10 minutes before slicing. Serve it immediately.