Mix rub ingredients in a small bowl and rub all over roast. Cover tightly with plastic wrap and refrigerate overnight.
Preheat oven to 225°F. Unwrap roast and bring to room temperature, about 40 minutes.
Mix mushrooms and onions and spread out on sheet pan.
Sprinkle champagne vinegar and sliced butter onto mushrooms/onion mixture.
Place roast fat-side up on top of mushroom bed, and roast in oven for 90 minutes, or until meat thermometer inserted into thickest part reads 110°F.
Remove from oven and rest at room temperature for at least 30 minutes, or up to 2 hours.
40 minutes before serving, preheat oven to 425°F.
Brush roast with melted butter and finish roasting for 20 minutes, or until meat thermometer inserted into thickest part reads 125°F.
Rest for 20 minutes before slicing. Serve with mushrooms and onions over the top and an extra drizzle of oil and smoked salt.