Preheat the oven to 200°C/400°F/ Gas Mark 6. Plunge the cauliflower into a pan of boiling water, bring back to the boil and cover and simmer for 5 minutes. Drain thoroughly, then transfer the cauliflower to an ovenproof dish and set aside. Melt the butter and flour together in a pan and cook, stirring over a low heat for 2 minutes. Remove the pan from the heat and gradually blend in the milk. Return to the heat and bring to the boil, stirring until thickened. Stir in the mustard and Boursin, then remove from the heat and allow to cool slightly.
Meanwhile, heat the oil in a large frying pan and cook the lardons or pancetta for 5 minutes. Add the breadcrumbs and cook for a further 5 minutes, stirring and scraping up any juices, until crisp and golden. Remove from the heat and allow to cool. Stir in the lemon zest, seeds and cheese. Pour the sauce over the cauliflower to cover and scatter over the breadcrumb mixture. Bake for 25-35 minutes, until golden and piping hot. Serve immediately