Soak the sultanas in the vinegar overnight – this not only plumps them but gives them a sweet and sour flavour.
Preheat the oven to 170–180°C.
Put the potatoes and onions into a high-sided baking tray or roasting dish. Mix in the garlic, bay leaves, thyme and olive oil, using your hands, and season. Add the stock or wine and water, then cover with foil and bake in the oven for around 20 minutes. When the potatoes are juicy and cooked, sprinkle over the sultanas and the toasted pine nuts.
Turn on your grill.
Put the cod on a baking tray, drizzle with olive oil, season with salt and pepper, then cover with alioli and put under the grill – it should take 4–5 minutes for a thinner portion of cod, 8–10 minutes if it is on the thicker side. Squeeze the sides of the fish, and if you can feel any rubberiness in the middle it isn’t cooked yet (you could also pierce it with a toothpick: if it goes through easily, it’s done; if there’s any resistance, then it isn’t).
Sprinkle the chervil on top of the potatoes and serve with the cod.