Heat your oven to 200°C/Fan 180°C/Gas 6. Add 1 tbsp oil to a roasting tin. Heat it in the oven for 10 mins while you prepare the celeriac.
Peel the celeriac, slicing off any knobbly roots. Cut the into 2cm slices then into 2cm-thick chips. Tumble carefully into the roasting tin. Cook for 40 mins, tossing halfway through.
Meanwhile, grate half the Parmesan into a bowl. Peel and grate or crush the garlic. Add to the bowl with 1 tbsp wholegrain mustard and half the thyme leaves. Smoosh together with a spoon, forming a thick paste. Set aside.
Peel and thinly slice the onions. Heat ½ tbsp oil in a small pan and add the onion. Season. Gently fry for 10 mins till soft. Sprinkle in 1 tbsp brown sugar and stir to dissolve. Pour in 1 tbsp balsamic vinegar and the remaining thyme leaves. Allow to bubble up, then turn the heat right down. Cover and gently cook for 10 mins while you cook the venison steaks.
Rub each steak with ½ tsp oil and some salt and pepper. Heat a griddle or frying pan till smoking hot. Fry the steaks for 2-3 mins on each side.
Heat your grill to its highest setting. Pop the steaks on a baking tray. Spread the Parmesan and mustard mixture onto the steaks and smooth all over with the back of a spoon. Slide the steaks under the grill for 2 mins till bubbling and golden.
Serve the mustard-crusted venison steaks with a spoonful of the sticky onion jam, celeriac chips and a handful of rocket.