Put the milk, quartered onion, mace and bay leaves in a pan, and bring to a simmer. Turn off the heat, set aside for 1 hr to infuse, then strain.
Meanwhile, bring a large pan of water to the boil. Tip in the cauliflower and cook for 5 mins until just tender. Drain and set aside, uncovered, in a colander to steam-dry.
Put the pan back on the heat, then add the knob of butter and the chopped onion. Gently soften onion for 10 mins until really soft but not coloured.
In a medium-sized pan, melt the butter, then stir in the flour to form a paste. Cook for 2 mins, then gradually stir in the strained milk to form a smooth sauce. Bring to the boil and simmer, stirring often, until thickened and smooth. Stir in the softened onion and season well with salt and nutmeg.
Heat oven to 220C/200C fan/gas 7. Tip the cauliflower into an ovenproof dish and pour over the sauce. Can be covered and chilled for up to 2 days at this stage or frozen. Scatter the cauliflower with the breadcrumbs, then bake for 30 mins until the sauce is bubbling and the top is golden and crisp.