Cook the cauliflower in a large saucepan of lightly salted boiling water for 2 minutes, then add the broccoli and cook for a further 3 minutes, until the vegetables are just tender. Drain well.
Meanwhile, place the sauce in a medium saucepan and cook over a moderate heat, stirring occasionally, until simmering. Stir in the sun-dried tomatoes, cauliflower and broccoli and mix together.
Transfer to a shallow ovenproof dish, sprinkle with the grated cheese and cook under a preheated hot grill for 2–3 minutes, until the cheese has melted.