The whole roast cauliflower is best served immediately that it is cooked, as the texture will suffer if it is prepared ahead and reheated. However the tomato mushroom sauce can be prepared in advance, and actually benefits from being made ahead as this allows the flavours to meld and intensify. Store the prepared sauce in an airtight container in the refrigerator for up to 3 days, then gently reheat it on the stove, thinning with a little water if necessary, before serving with the freshly roasted cauliflower.