Heat oven to 200C/180C fan /gas 6. Mix the butter in a bowl with the garlic, parsley and lemon zest. Scrape out onto a piece of cling film, roll into a sausage and keep in the fridge until needed.
Toss the spiralized celeriac on a large baking tray in 1⁄2 tbsp oil, rosemary and some seasoning then spread out the celeriac, leaving enough room between the ‘fries’ so that they cook evenly. Roast in the oven for 20 mins until tender and crisped.
Heat a large frying pan over a high heat while you season the steaks. Once searing hot add the remaining oil and the steaks. Cook for 2 mins on each side for medium rare, then set aside on a plate to rest.
While the steak is resting, put the kale in a large microwaveable bowl with 50ml water. Cover then cook on high for 3–4 mins, or until wilted. Slice the butter into rounds. Serve on top of the steak and kale alongside the celeriac fries.