Arrange the tomatoes in the base of shallow ovenproof dish. Steam the spinach for 1–2 minutes until just wilted and spoon over the tomatoes. Sprinkle with a little nutmeg and some seasoning.
Steam the cauliflower for 8–10 minutes until just tender. Meanwhile, melt the butter in a separate saucepan and stir in the flour. Gradually whisk in the milk and bring to the boil, stirring constantly, until smooth and thick. Stir in two-thirds of the cheese and season well.
Arrange the cauliflower on top of the spinach, then gently mix together. Pour the sauce over the top. Mix the remaining cheese with the torn bread and seeds then sprinkle over the cheese sauce. Allow to cool, cover and chill until required.
When ready to serve, remove the cover and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until the topping is crisp and golden and the vegetables are piping hot.