Method
Prep:25min  ›  Cook:20min  ›  Ready in:45min 

1. Preheat the oven to 190°C/gas 5. Trim away the leaves from the cauliflower, then cut off the florets from the main stem. Cut the broccoli florets away from the stem. Blanch both the cauliflower and broccoli florets in a large saucepan of boiling water for 1 minute, then drain thoroughly.
2. Put the spring onions into the saucepan. Mix the cornflour with a little of the milk in a heatproof bowl to make a smooth paste. Add the rest of the milk to the onions and bring to the boil. Stir half of the hot milk into the cornflour paste, then return the mixture to the pan. Bring to the boil, stirring frequently, to make a smooth sauce.
3. Remove the sauce from the heat and stir in the grated cheese, beaten egg and half the breadcrumbs. Stir in the blanched vegetables and season to taste.
4. Transfer the mixture to a greased, ovenproof baking dish or gratin dish. Scatter the rest of the breadcrumbs and the walnuts over the top, then bake for 20 minutes until golden. Serve immediately.

variations

Pine nuts are a good alternative to walnuts. * You could also arrange 100g halved baby plum tomatoes, cut-side up, on top of the vegetables in the sauce, before adding the breadcrumbs and nuts.