Preheat the oven to 200°C (180°C fan, gas mark 6).
Scrub the beetroot, skin on, and trim both ends. Place it on a large sheet of greaseproof paper, add in the thyme, garlic cloves and a good splash of olive oil. Season with salt and pepper.
Wrap everything in the paper, then an equally large piece of tin foil. Seal and place into a baking dish. Bake for 1 hour, or until beetroot is soft, but take the garlic out after 50 minutes.
Halve the onions, cut off the bottoms but leave the skin on. Put the onions into a baking tray, skin-side-up. Drizzle over with olive oil and salt flakes and pop into the oven for 35 minutes.
You can cook both the beetroot and the onions at the same time. Once softened, remove from the oven, and keep aside.
To prepare the prunes, put the ingredients into a wide pan, except the prunes, and bring to the boil. Add in the prunes. Allow everything to start boiling again for no more than half a minute, then take the pan off the heat.
Leave the prunes to sit in the syrup.
Turn the oven up to 220°C (200°C fan, gas mark 7).
Rub a little oil, salt and pepper over the venison and place the meat in an ovenproof frying pan or cast-iron skillet. Heat upon the hob until the venison browns all over.
Transfer the dish into the oven for 5-10minutes, then remove and allow to rest for another 10 minutes.
Peel the beetroot, garlic cloves and the onion, then cut the beetroot and the onion into rough pieces.
Slice the venison steak and arrange everything on a serving platter.
Deglaze the venison cooking pan with 4-6tbsp of the prune cooking liquid and drizzle over the salad.
Season to taste with salt and pepper. Roughly chop the spring onions and scatter over the salad with the parsley and a good grating of horseradish.