Butterfly the backstrap and pound between 2 pieces of waxed paper until 3/4-inch (2cm) thick.
Marinate in the Caesar salad dressing overnight.
Prepare a grill or oven for 375 degrees F (191 degrees C).
Prior to cooking, let the marinated venison sit at room temperature for 30 minutes. Remove the backstrap from the marinade and place it on a prepared plank or ovenproof baking dish.
Mix all the remaining ingredients, except the spinach. Spread the Parmesan mixture evenly over the backstrap. Lay the spinach over the Parmesan mixture and fold over at least one time. Coat the venison with the remaining Caesar dressing.
Grill or bake for 20 to 25 minutes or until the internal temperature reaches 165 degrees F (74 degrees C). Let sit at least 5 minutes before slicing.