Preheat oven to 375°. Toss potatoes and onion with salt, pepper, olive oil. Add to sheet pan, place bacon next to potatoes. Cook 15 min. remove bacon (chop and reserve). Push potatoes to one side of sheet pan. Add Brussels sprouts next to potatoes, tossing with oil, salt and pepper. Return pan to oven until potatoes are done and sprouts are soft, 25 minutes. Remove sprouts to serving platter. Roughly smash potatoes on pan with onion. Add chopped bacon, crumbled cheese and ¼ cup melted butter, return to oven and broil until browned, 2 minutes. Serve immediately. Meanwhile, season steak well with salt and pepper; sear in hot cast iron skillet 2 minutes. Flip steak and add crushed garlic, thyme, and 2 T butter, basting until cooked through (drizzle with some bacon drippings if desired). Let rest, slice, and serve with pan drippings.