Preheat the oven to 350°F.
In a small pot, combine the port, stock and peppercorns. Bring to a boil over high heat. In a small bowl, whisk the cornstarch with about a tablespoon of water until smooth. Whisk the cornstarch mixture into the pot. Continue to boil until a nice thick sauce forms, about 15 minutes longer. Once the sauce has reached your desired consistency, stir in the butter to give the sauce a nice shine. Remove the peppercorns and taste, adding more salt if desired.
Season the venison with salt and pepper. Heat the olive oil in an oven-safe pan over medium-high heat. Add the venison and sear on all sides to brown. Transfer the pan to the oven and cook for approximately 8 minutes more or until an internal meat thermometer reads 145°F. Remove from the oven and cover with foil. Allow the meat to rest for a few minutes before slicing.
When ready to serve, slice the venison crosswise so you have nice rounds of tenderloin. I like to serve this with some polenta and seasonal mushrooms. Drizzle generously with the sauce.