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Heat oven to 150C/130C fan/gas 2. Melt the butter in a large heavy-based frying pan. Add all the vegetables (or do this in 2 batches if necessary rather than overcrowd the pan) and a little sea salt and freshly ground black pepper, then sweat them on a medium heat for 5 mins.
Transfer the veg to a large roasting tin, pour in 150ml water, cover with foil, then roast for about 2 hrs or until meltingly tender but still holding their shape. Taste and correct the seasoning.