Once the rock is hot enough that a drop of water sizzles and vaporizes, brush on a film of flavored olive oil, such as chipotle, then add backstrap medallions and 1⁄4-inch-thick slices of onion. Season with cracked black pepper. Cook the venison and onions for 2 to 3 minutes. Using tongs, remove them to a plate. Turn the rock over, brush away the ashes, and drizzle with more flavored olive oil. Place the medallions and onions on the fresh, hot surface (2), cooked side up, and top each medallion with an oil-packed sun-dried tomato. Cook another 2 minutes. Stack an onion slice on the meat and sun-dried tomato, and dig in.