Heat the oven to 150C (non- fan 170C). Trim and thickly slice the leeks and spring onions, peel and finely slice the garlic, then pick the thyme leaves.
Place a large pan over a medium heat and add half of the butter with a splash of oil. Add the thyme leaves, bay, leeks, spring onion and garlic and cook for 10 to 15 minutes, or until starting to catch and turn golden.
Spread the softened veg in a large roasting tray. Using a small knife, slash the fish four times on each side, then place on the leeks.
Bash and sprinkle over the fennel seeds and season well with sea salt and black pepper. Pour the cider around the fish and dot with the rest of the butter. Bake for 1 hour. Transfer the fish to a dish, cover with tin foil and leave to rest.
Place the tray with the leeks on the hob over a medium-low heat, then add the mascarpone and let it bubble away for a few minutes, or until thickened. Season.
Serve the whole roasted turbot on the bed of leeks. Slice and arrange the lemon on top, chop and scatter over the parsley leaves, then tuck in.