Pumpkins and squashes herald the start of autumn, when thoughts turn to cooking warming, nourishing food. This is when these versatile ingredients shine. Varieties are countless, but worth a special mention are crown prince, butternut and – my favourite – delica, with its dense, intensely sweet, rich and buttery flesh. Always look for bright skin without blemishes and ensure it is firm to the touch. Gratins are the ultimate one-pot wonder. A good one delivers bags of pleasure and this dish is no exception. Here, the flavour that comes from the squash ties in brilliantly with the mustard and the sage.