1. Remove the venison from the fridge and leave for 30 minutes to let the meat reach room temperature.
2. Make small incisions evenly across the venison and insert a garlic piece or rosemary sprig in each hole.
3. Preheat the oven to 230℃(450°F).
4. Fry the onions in the butter until softened.
5. Arrange the onions in the centre of a roasting dish.
6. Place the venison on top of the onions and pour the oil over the meat, ensuring to cover all areas of the meat.
7. Season liberally with salt and pepper.
8. Roast the meat for 20 minutes in the oven, then remove from the heat.
9. Lower the oven setting to 180℃(350°F)
10. Meanwhile, baste the venison with the oil and juices collected at the bottom of the pan.
11. Roast the meat for 25 minutes and check the meat’s internal temperature with a meat thermometer.
12. When the meat has been cooked to desired doneness, let it rest for at least 20 minutes.
13. While the meat is resting, pour the onions and cooking juices in a saucepan.
14. Blend the contents in a food processor until smooth.
15. Serve slices of venison with typical Sunday roast trimmings.