Preheat the oven to 190 degrees C. 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.
Put the spring onions into the same saucepan. Mix the cornflour with a little of the milk in a large heatproof bowl to make a smooth paste. Pour the rest of the milk into the saucepan with the spring onions and bring the milk to the boil. Stir half of the hot milk into the bowl with the cornflour paste, mix well, then return the entire mixture to the saucepan. Bring the mixture to the boil, stirring frequently, to make a smooth white sauce.
Remove the white 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 with freshly ground black pepper.
Transfer the vegetable mixture to a greased, ovenproof baking dish or gratin dish. Scatter the rest of the breadcrumbs and the walnuts over the top of the vegetables, then bake for about 20 minutes until golden. Serve immediately.
Used different ingredients.
As I had no walnuts and find almonds more palatable anyway, I used flaked almonds for the topping - and also added a sprinkling of grated parmesan.
Would also suggest using strong vintage cheddar, for a well-flavoured sauce. - 27 Jan 2010