Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the skin from the celeriac, slice off the roots and chop the celeriac into 2cm cubes. Tumble onto a baking tray with 1 tbsp olive oil and some seasoning. Roast for 30 mins till golden and tender.
While the celeriac is roasting, prepare your mushroom sauce. Trim the leeks and slice them in half lengthways. Rinse out any grit. Thinly slice them. Thinly slice the mushrooms. Peel and grate or crush the garlic. Finely chop the parsley stalks and leaves, keeping them separate.
Place the dried porcini in a bowl and cover with 200ml boiling water. Leave to one side to soak.
Set a frying pan on the medium heat for a few mins. When warm, drizzle in 1 tbsp oil and add the leeks and sliced mushrooms. Season. Fry for 5 mins, stirring, till softened and juicy. Add the garlic and fry for 1 min. Transfer the cooked leeks and mushrooms to a bowl and wipe the pan clean with kitchen paper.
When the celeriac has 5 mins left to cook, season the venison steaks and rub each one with 1 tsp oil. Place the pan back on the heat. When smoking hot, lay the steaks in the pan. Fry for 3-4 mins on each side, or longer if they are thicker than 2cm, or if you prefer a more well-done steak. Lift the steaks on to a warm plate and loosely cover with foil to rest.
Tip the mushrooms and leeks back into the pan on a medium heat. Pour in 1 tbsp balsamic and bubble for 1 min. Pour in the porcini and their stock and add the parsley stalks. Simmer for 3 mins.
While the sauce is bubbling, tip the roast celeriac into a food processor. Add 1 tbsp Dijon mustard. Whizz till smooth. No processor? No problem, just crush the celeriac with a fork or masher, stirring through the Dijon. Taste and adjust the seasoning.
Add 4 tbsp double cream to the mushroom sauce. Stir, taste and adjust the seasoning. Lay the steaks in the sauce and turn them a couple of times to warm them a little.
Spoon the celeriac mash on to a couple of warm plates. Top with a venison steak each, spooning over the rich mushroom sauce. Scatter over chopped parsley leaves to serve.