1: A day ahead, preheat the oven to 110°C/Gas Mark ¼. Place the Spanish onions in a roasting tray, give them a little drizzle of rapeseed oil and add a sprinkling of thyme leaves. Place the tray into the oven and roast the onions for 8 hours.
2: Remove the tray from the oven and leave the onions to cool a little. Scoop out the roasted onion flesh and chop it to form a rough onion paste. Place this into a colander or fine sieve and leave to drain for 3–4 hours.
3: Place the soft brown sugar in a saucepan over a high heat and stir until it starts to melt. Add the balsamic vinegar, bring to the boil and boil for 1–2 minutes, without stirring, until a caramel forms. Add the drained onion paste and heat through. Stir in the onion seeds and the chives. Season. Remove the pan from the heat and leave to one side until needed.
4: Meanwhile, place the white onion rings in a bowl and pour over the milk and leave to one side.
5: Preheat the oven to 190°/Gas Mark 5. Mix the sea salt flakes, demerara sugar and remaining thyme leaves together, then sprinkle a thin layer of this mix on the base of a roasting tray. Place the carrots on top then cover with the rest of the salt mix. Place the tray in the oven and roast the carrots for 45 minutes, or until they are tender. Remove the tray from the oven and leave the carrots to cool a little.
6: When the carrots are cool enough to handle, scrape the salt mix off them. With a sharp knife scrape the skin from the carrots and keep them hot in the turned-off oven.
7: Remove the onion rings from the milk, pat dry and lightly dust them in the flour, shaking off any excess. Heat enough oil for deep-frying in a deep-fat fryer or heavy-based saucepan until it reaches 180°C. Add the onion rings and fry for about 2 minutes until crispy and golden brown. Drain on kitchen paper and season. Leave to one side and keep hot in the turned-off oven (but do not cover).
8: Place the wine, beef stock and Marmite together in a frying pan over a high heat and bring to the boil. Continue boiling until the liquid has reduced by one-third. Turn the heat down to very low and add the plaice fillets, then turn the heat off and leave them to poach in the residual heat for 4–5 minutes until they are firm to the touch.
9: Turn the fish over in the wine to finish cooking for 30 seconds. Remove the fillets from the wine and keep warm. Bring the wine back to the boil and boil until reduced by half. Pass this through a fine sieve into a clean pan and put back onto the heat. Gently stir in the butter.
10: Serve the poached plaice fillets with the slow-roasted onions and salt-baked carrots. Pour over a little sauce and then add the crispy onion rings. Serve immediately.