1. To make the sauce, heat 1 tbsp of the butter in a large frying pan, tip in the onion and cook over a low heat for about 10 mins until soft.
2. Add the garlic and mushrooms, increase the heat and fry until all the liquid has evaporated.
3. Meanwhile, heat enough oil to come 4-5in up the sides of a large saucepan. When hot, add the chips and cook, in batches, for 3-5 mins until golden. Remove and drain on kitchen paper, but keep the oil hot.
5. Heat the remaining butter in a frying pan over a medium-high heat. Season the venison and cook for 3 mins on each side for medium-rare, depending on the thickness of the meat. Remove from the pan and leave to rest, loosely covered with foil.
6. Tip the fries back into the hot oil and cook for a further 2-3 mins to crisp up. Drain and season with salt. Serve the steaks with the sauce poured over and a pile of fries on the side.