Heat the oven to 220°C/Fan 200°C/Gas 7. Fill the kettle and boil it. Wash and halve the celeriac. Peel it. Slice the celeriac into chunky chips. Rinse the rosemary sprigs. Pat dry. Pick the leaves from the stalks. Finely chop them.
Fill a pan with water from the kettle. Add the celeriac chips. Bring to the boil. Simmer for 3 mins.
Drain the celeriac. Spread out on a baking tray. Drizzle with olive oil. Season. Sprinkle with half the rosemary and a pinch of the chilli flakes. Bake for 30 mins till golden brown. Shake half way through.
Rinse and slice the mushrooms. Season the steaks with salt and pepper. Warm a splash of olive oil in the frying pan.
Add the steaks to the frying pan and fry for 1-2 mins each side on a high heat. Pop over a plate to rest. Cover with foil.
Add the mushrooms to the frying pan with a pinch of salt and pepper. Stir and fry for 4-5 mins till golden.
Rinse the salad leaves. Drain. Dry in a salad spinner or pat dry with kitchen paper. Pop into a bowl. Splash with a little olive oil.
Add the balsamic vinegar and the remaining rosemary to the mushrooms. Sizzle for 1 min. Serve with the steaks, salad and celeriac chips.