Cheesy Cauliflower and Ham Gratin

Serves 4

Cut the cauliflower into florets, reserving the small green leaves. Cook in a pan of boiling, lightly salted water for 8 minutes, until just tender. Add the leaves and cook for a further 2 minutes. Drain well and tip into a deep, 2-litre heatproof dish.
Meanwhile, preheat the grill to high. Make the sauce. Melt the butter in a pan over a medium heat. Add the flour and cook, stirring, for 1 minute. Gradually stir in the milk, bring up to a simmer and cook, stirring, until thickened. Stir in 100g of the cheese, the mustard and half the ham. Season with freshly ground black pepper.
Pour the sauce over the cauliflower and scatter with the remaining cheese and ham. Pop under the grill for 3-4 minutes, until the cheese is melted and golden.