Begin by toasting the cumin and coriander seeds in a small saucepan over medium heat until fragrant, then crush them lightly in a pestle and mortar.
Scrub the beetroot and cut them into wedges. Spread them across a baking tray and drizzle with half of the olive oil. Scatter over half of the crushed spices and half of the black onion seeds. Tear over half of the rosemary and season generously with salt and black pepper. Roast in a preheated oven for 45 minutes to 1 hour, turning once or twice.
While the beetroot roasts, mix the remaining spices into the yoghurt along with the grated garlic and 1 tablespoon of olive oil.
Season the venison steaks with salt, pepper, and the chilli flakes. Heat a medium pan over high heat, add the remaining olive oil, and once hot lay in the steaks. Cook for 1–2 minutes on the first side, then flip and add the butter, the whole garlic clove, and the remaining rosemary. Baste the steaks with the bubbling butter as they finish cooking, then remove them from the pan to rest.
Keeping the pan on the heat, add the walnut halves to the garlicky butter and toast them for 3–4 minutes. Remove the pan from the heat.
To serve, arrange the roast beetroot across a warm platter. Slice the venison thickly and place it around the beetroot, spoon over any resting juices, drizzle with the spiced yoghurt, and finish with the toasted walnuts.
