1. Cooking indoors:
Preheat your oven to 180°C/Fan 160°C/Gas 4. Remove all packaging and string from the meat and pop into a roasting tin. Rub with 1 tbsp olive oil and some salt and pepper. Let the joint sit for 30 mins to come to room temperature.
2. Slide into the hot oven and roast for 40 mins for medium-rare, or add 10-15 mins for a more well done haunch. Pop on a plate to rest for 30 mins.
3. While the venison rests, halve and stone the plums and slice the flesh into small pieces. Peel and finely chop the red onion. Pick the leaves from the thyme sprigs.
4. Pour 1 tbsp of oil into a pan and warm it to a medium heat. Add the onion and thyme leaves along with a pinch of salt and pepper. Fry for 2 mins, till softened, then add the plums and stir in 1 tbsp each of the sugar and mustard, and 2 tbsp of cold water. Cover and reduce to a low heat and cook for 10 mins, stirring occasionally, till the plums have started to dissolve into a sauce.
5. Serve the venison in thin slices, with the plum sauce on the side.
6. Cooking Outdoors:
Light your barbecue and let it cool to a medium heat. Unwrap the meat and unroll it to flatten out. Rub with 1 tbsp olive oil and a good pinch of salt and pepper.
7. Place the joint on the hottest part of the barbecue and cook for 2-5 mins each side, till coloured. Move to the coolest part and cover. Barbecue for 30-35 mins, then pop on a board. Rest for 20 mins.
8. Meanwhile, halve and stone the plums and slice the flesh into small pieces. Peel and finely chop the red onion. Pick the leaves from the thyme sprigs. Take a large square of kitchen foil and add the plums, red onion, sugar, mustard, thyme, 1 tbsp oil, 2 tbsp of cold water and a pinch of seasoning. Toss together and fold the foil into a loose parcel. While the meat rests, pop the parcel onto the coolest part of the grill. Cook for 15 mins, till the plums break down into a sauce.
9. Serve the venison in thin slices, with the plum sauce on the side.