In a small bowl, combine crushed juniper berries, chopped rosemary, minced garlic, salt, and pepper. Rub this mixture all over the venison medallions. Allow the meat to marinate for at least 30 minutes at room temperature.

Heat olive oil in a skillet over medium-high heat. Once hot, add the venison medallions and sear for about 3-4 minutes on each side, or until they reach your desired level of doneness (medium-rare is recommended). Remove the medallions from the skillet and let them rest.

In the same skillet, lower the heat to medium and add the blackberries. Cook for 2-3 minutes until they begin to break down. Pour in the beef or venison stock and balsamic vinegar. Bring to a simmer and let it reduce for about 5-7 minutes.

Stir in the chopped thyme and unsalted butter, mixing until the butter is melted and the gravy is smooth. Season with salt and pepper to taste.

Slice the rested venison medallions and place them on a serving plate. Drizzle the blackberry and thyme gravy over the top and serve immediately.