Pat dry a fully thawed, room temperature venison backstrap with paper towels. Slice the onions and add to a skillet with butter over medium heat. Sauté for about 5 minutes. Do not fully cook the onions.
Coat all sides of the backstrap with the wet rub. Cut the backstrap in half to fit in the skillet. Nestle the backstrap in the cooked onions searing on both sides for 2-3 minutes per side.