Heat your oven to 200C/Fan 180C/Gas 6. Add 1 tbsp olive oil to a roasting tin and slide into the oven to heat up.
Peel the potatoes and chop them into bite-size chunks. Add them to the tin and turn to coat in the oil. Season with salt and pepper and roast for 10 mins.
While the potatoes roast, peel and halve the red onions. Rub 1/2 tsp oil into the cut side of each onion. Season with a little salt and pepper. When the potatoes have cooked for 10 mins, add the onions to the tin and roast for 30 mins till the potatoes are golden and the onions tender.
Peel and finely chop the shallots. Add them to a small pan with 2 tbsp water. Season with a pinch of salt and pepper and place over a low heat. Gently cook, stirring now and then, for 5-8 mins till the shallots are soft. If they start to catch and burn, add more water and keep the heat low.
While the shallots cook, peel and finely chop the garlic. Finely chop most of the rosemary leaves. Stir the garlic and rosemary into the shallots. Add the blueberries. Cover and gently cook for 5-8 mins till the blueberries have burst and formed a rough sauce. Stir now and then. Take off the heat and leave to stand with the lid on.
Rub 1 tsp oil and some salt and pepper into each venison steak. Heat a griddle or frying pan till it's smoking hot. Add the steaks and fry for 2-3 mins. Flip and fry for 2-3 more mins (this should give you medium steaks. If you prefer more well done meat, or you have thick steaks, cook for a 2-3 mins more each side).
Pop the steaks on a warm plate. Loosely cover with foil and rest for 5 mins. Once they've rested, tip any any juices into the blueberry sauce. Pop the steaks on two warm serving plates and serve with the roast potatoes, red onions, a handful of watercress and the blueberry sauce.