As the celeriac roasts, it absorbs some of the raisiny flavour of the marsala (but not the alcohol, which just burns off), while caramelising to a golden, sticky brownness. Excellent with game, in particular.
Preheat the oven to 180C/350F/Gas 4.
Place the celeriac wedges into a large bowl, drizzle with a little oil and stir to coat them.
Smear the butter thickly around an ovenproof dish, just large enough to take the celeriac wedges lying down flat. Lay the celeriac in the dish, season with salt and freshly ground black pepper and pour over the marsala.
Transfer to the oven and roast for about an hour, turning the wedges and basting every now and then, until richly browned all over and very tender. You may need to add a little water towards the end of the cooking time to prevent burning. Serve warm.