Season the venison tenderloin generously with salt and pepper. Let it rest at room temperature for about 10-15 minutes.

In a large skillet, heat the olive oil over medium-high heat. Once hot, add the venison and sear for about 3-4 minutes on each side until you achieve a nice brown crust. The internal temperature should reach about 130°F for medium-rare doneness.

Once cooked, remove the tenderloin from the skillet, cover it with foil, and let it rest for about 10 minutes.

In the same skillet, add the chopped onion and minced garlic. Sauté until the onion is translucent and fragrant.

Add the fresh blackberries and beef stock, scraping the bottom of the pan to incorporate all the flavors.

Bring the sauce to a simmer and let it cook for about 10 minutes, or until it reduces and thickens slightly.

Add in the fresh thyme during the last few minutes of cooking for added flavor. Adjust the seasoning with salt and pepper.

Thinly slice the rested venison tenderloin and plate it. Drizzle the blackberry reduction over the top and garnish with additional blackberries, if desired.