Preheat the oven to 200C/Gas 6.
In a roasting tray scatter in the potatoes, celeriac, onion, garlic and thyme. Drizzle with olive oil and season with a good pinch of salt and pepper, then add a few small dollops of butter to the pan. Give the pan a good shake so that everything is gloriously coated in seasoning and oil. Place in the oven and roast for 25 minutes or until everything has turned lightly golden.
While the veg cooks, make the garlic butter. In a small bowl add the butter, grated garlic, parsley and a good pinch of salt. Mix everything together and set aside.
Remove the pan from the oven and give everything a good shake. To the pan scatter in the mushrooms. Season the gurnard fillets and lay them carefully on top of the veg. Give the pan another light drizzling with oil and then dollop in the garlic and parsley butter, making sure you give the gurnard fillets a decent covering. Place the pan back into the oven and roast for a further 12 minutes.
Remove from the oven and serve straight away.