For the winter vegetable ratatouille, preheat the oven to 200C/400F/Gas 6.
Mix all of the vegetable cubes, the onion and the garlic in a deep-sided roasting tin until well combined.
Drizzle over some of the olive oil and season, to taste, with salt and freshly ground black pepper.
Place the roasting tin over a medium heat and stir the vegetables well to coat all of the vegetables in the oil.
When the vegetables start to sizzle, transfer the roasting tin to the oven and cook for 35-40 minutes, stirring the vegetables occasionally and adding more of the remaining oil, as necessary, until the vegetables are tender.
Meanwhile, for lemon beurre blanc, bring the shallots, white wine vinegar, lemon juice, lemon zest and white wine to the boil in a non-reactive saucepan. Continue to boil until the volume of liquid has reduced to two tablespoons.
Add the cold water and return the mixture to the boil. Boil again until only one tablespoon of liquid remains in the pan.
Reduce the heat to low, then whisk in the butter, a cube at a time, waiting until each cube has melted before adding the next. Continue to whisk until the sauce is pale and thick, then remove the pan from the heat and season, to taste, with salt and freshly ground black pepper. Add more lemon juice, as necessary, to taste.
Strain the sauce through a sieve into a clean saucepan and set aside until needed (do not refrigerate or the sauce will separate). Gently reheat the sauce just before serving, stirring well.
Meanwhile, for the roast cod, reduce the oven temperature to 180C/350F/Gas 4.
Heat the oil and butter in a frying pan over a medium heat. When the butter has stopped foaming, place the cod fillets into the pan, skin-sides down, and fry until the skin is crisp and golden-brown. Season, to taste, with salt and freshly ground black pepper. Carefully turn the fillets over and continue to fry until golden-brown on both sides.
Transfer the cod fillets to a roasting tray and cook in the oven for 4-5 minutes, or until just cooked through. (NB: The cod is cooked through when the flesh is opaque.)
To serve, divide the winter ratatouille equally among six serving plates. Place one roasted cod fillet on top of each serving. Spoon over the lemon beurre blanc.
