1 kg (2 lb) mixed vegetables (courgettes, carrots, leeks, mushrooms), cut into bite- sized pieces
500 g (1 lb) potatoes, peeled and cut into chunks
50 g (2 oz) plain flour
575 ml (18 fl oz) milk
2 teaspoons Dijon mustard (optional)
200 g (7 oz) mature Cheddar or Double Gloucester cheese, grated
salt and pepper
Melt half the butter in a large, heavy-based casserole or saucepan and gently cook the mixed vegetables for about 20 minutes, stirring occasionally, until tender and golden.
Cook the potatoes in a large saucepan of lightly salted boiling water for 10–12 minutes, until just tender.
Meanwhile, place the remaining butter, the flour and the milk in a saucepan over a medium heat, and stir constantly with a balloon whisk or wooden spoon until thickened. Simmer gently for 1–2 minutes, then stir in the Dijon mustard, if using, and half the grated cheese. Season lightly to taste and set aside.
Transfer the cooked vegetables to a large ovenproof dish and pour over the cheese sauce. Sprinkle with the remaining cheese and cook under a preheated grill for 5–7 minutes, until bubbling and golden. Serve immediately.