Heat half the butter in a frying pan, add the cauliflower and broccoli and fry until just tender. Transfer to an ovenproof dish.
Melt the remaining butter in a saucepan, add the cornflour and cook, stirring, for 1 minute. Gradually add the milk and cook, stirring all the time, until thickened and smooth, then season well. Stir in two-thirds of the Gruyère, then crumble in half the bacon.
Pour the sauce over the vegetables. Mix together the remaining Gruyère and the breadcrumbs and sprinkle over the top with the remaining bacon.
Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–12 minutes until golden and bubbling.