750 g (1 1/2 lb) butternut squash, peeled, deseeded and thinly sliced
2 tablespoons chopped parsley
3 tomatoes, thinly sliced
150 ml (1/4 pint) vegetable stock
25 g (1 oz) plain flour
300 ml (1/2 pint) milk
1/2 teaspoon freshly grated nutmeg
1 teaspoon Dijon mustard
150 g (5 oz) Emmental or Gruyère cheese, grated
Melt 50 g (2 oz) of the butter in a large, heavy-based frying pan or wok and cook the onions and squash over a medium-high heat, stirring occasionally, for 5 minutes until beginning to soften and brown in places. Add the parsley, tomatoes and stock and bring to the boil. Reduce the heat, cover and simmer for 5 minutes.
Meanwhile, melt the remaining 25 g (1 oz) butter in a saucepan, add the flour and cook over a medium heat, stirring, for a few seconds. Remove from the heat and add the milk, a little at a time, stirring well between each addition. Return to the heat, then bring to the boil, stirring constantly, cooking until thickened. Remove from the heat, add the nutmeg, mustard and half the cheese and stir well. Add to the pan with the squash and toss together.
Transfer to a large gratin dish, sprinkle with the remaining cheese and cook under a preheated medium grill for 10 minutes until golden and bubbling. Serve with crusty bread and a simple salad.