Melt the butter in a pan then stir in the flour. Cook, stirring, for 3 minutes then gradually whisk in the milk to give a smooth sauce. Add the garlic, simmer for 4 minutes, then fish out. Add the mustard, cheddar and gruyère, and stir until melted. Season with salt and white pepper.
Heat the oven to 220C/fan 200C/gas 7. Cook the cauliflower wedges in a large pan of boiling salted water until just tender. Scoop out with a slotted spoon and drain really well.
Cook the macaroni in the same water following pack instructions. Drain, then rinse under cold water and drain again really well. Carefully mix the macaroni and cauliflower with the sauce and tip into a buttered 20cm x 30cm ovenproof dish.
Scatter over the grana padano or parmesan and bake for 20-25 minutes or until golden and bubbling. Leave to sit for 5 minutes before serving.