This hearty vegetable gratin makes for a delicious meat-free midweek meal or a great little side dish for a Sunday roast

Preheat the oven to 200C/Fan 180C/Gas Mark 6. Boil the potatoes in a large pan of lightly salted water for 8-10 minutes until almost tender, adding the broccoli to the pan for the last 2-3 minutes of cooking time. Drain well and place in a large shallow ovenproof dish.

Heat the oil in a large pan over a high heat and fry the onion for 3-4 minutes until just softened. Scatter the onion over the potatoes and broccoli. 

Melt the butter in the pan, then stir in the flour and cook for 1 minute until foaming. Remove the pan from the heat and gradually stir in the milk. Return to the heat and bring to the boil, stirring all the time. Reduce the heat and simmer for 1-2 minutes, still stirring, until the sauce is smooth and thickened. 

Stir in the half the cheese and the mustard and simmer for a further minute. Season to taste with salt and freshly ground black pepper. Pour the sauce over the vegetables in the dish and scatter over the rest of the cheese. Bake in the preheated oven for 30-35 minutes until golden and bubbling. Serve garnished with the basil leaves. 

Get the right spuds: Choose a firm, waxy-fleshed potato such as Charlotte or Desiree for this dish, or try sweet potatoes instead for a different flavour.