Heat your oven to 220°C/Fan 200°C/Gas 7. Halve the celeriac. Peel it, slicing off the roots on a diagonal. Slice the celeriac into chunky chips.
Tip the celeriac chips into a large pan. Cover with plenty of boiling water. Put a lid on the pan and bring to the boil. Simmer for 3 mins. While the celeriac simmers, pick the leaves from the thyme sprigs.
Drain the celeriac. Pop in a bowl. Add 2 tsp oil and most of the thyme leaves. Measure out 1 tsp cumin seeds and add them to the bowl. Season with salt and pepper. Spread out on a baking tray. Bake for 30 mins till golden brown.
Slice the mushrooms. Season the steaks with salt and pepper. Warm 1 tsp oil in a non-stick griddle or frying pan.
Add the steaks to the frying pan and fry for 1-3 mins each side over a high heat, depending on how well done you like them. Pop on a warm 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.
Toss the salad leaves with ½ tbsp balsamic vinegar and some seasoning. Add another ½ tbsp balsamic vinegar and the remaining thyme leaves to the mushrooms. Sizzle for 1 min. Serve with the steaks, salad and celeriac chips.