To make the stock, pour 3 litres/5¼ pints water into a large saucepan and add the carrot, celery, onion and parsley along with the reserved fish bones. Season generously with salt and simmer over a medium heat for 30 minutes, or until the volume of liquid has reduced by a third. Keep warm over a low heat.
Preheat the oven to 220C/200C Fan/Gas 7.
To make the timbale, grease a non-stick 25cm/10 inch fluted ring cake tin with a drizzle of olive oil.
Melt a knob of the butter in a heavy-based saucepan, add the shallot and cook over a medium heat until soft. Add the rice and cook, stirring, for 2–3 minutes, or until translucent.
Add a ladleful of hot stock and continue to cook over a medium heat, stirring until all the stock has been absorbed. Keep adding ladlefuls of stock, stirring constantly, allowing each ladleful to be absorbed before adding the next, until the rice no longer has a pale core and is soft but retains slight bite (approximately 15 minutes). Remove the pan from the heat and add the Parmesan, pepper and a knob of butter. Set aside.
In a small saucepan, melt the remaining butter and whisk in the flour to form a roux. Whilst stirring, slowly add the nutmeg, cream, Prosecco and enough stock to make a smooth, pale gravy. Simmer gently for five minutes over a low heat to thicken slightly, then remove from the heat and set aside.
Press the sole fillets against the bottom and sides of the prepared cake tin to line it, leaving the fillets overhanging slightly at top. Season well.
Spoon the risotto into the mould, pressing it down with the back of a spoon to pack as tightly as possible, then pour over half of the Prosecco gravy. Set aside the rest of the gravy.
Transfer the tin to the oven and bake for 15–20 minutes, or until lightly golden-brown and set. Remove from the oven and leave to rest for 30 minutes.
To serve, turn out the timbale onto a large serving dish. Scatter over the zest strips and serve with the remaining gravy.