Preheat the oven to 200°C.
Toss the celeriac cubes with salt, pepper, and chilli powder in a bowl.
Heat 30ml olive oil in an ovenproof dish until very hot.
Add the celeriac to the hot oil and roast for 20 minutes until golden.
Season the steaks with salt and pepper and rub with vegetable oil.
Sear the steaks in a hot pan for 2-3 minutes per side.
Sauté shallots, mushrooms, and garlic in butter until softened.
Stir in the tomato purée and cook for 3-4 minutes.
Pour in 50ml wine and simmer until reduced by a third.
Add orange zest, juice, tarragon, and 150ml stock and bring to a boil.
Simmer the liquid until reduced by two thirds.
Stir in a splash of orange liqueur just before serving.
Serve the steak with roasted celeriac and the orange sauce.