Heat oven to 190C/170C fan/gas 5. Toss the cauliflower with the oil and half the thyme, and spread out on a large non-stick baking tray. Roast for 20 mins until softened and golden brown. Meanwhile, bring a large pan of salted water to the boil and cook the macaroni following pack instructions. Drain.
Melt the butter in a medium saucepan, stir in the flour and mustard, and cook for 2 mins. Remove from the heat and gradually whisk in the milk and remaining thyme. Return to the heat and cook for 4-5 mins, whisking all the time, until you have a thick, smooth sauce. Remove from the heat for 2 mins, then stir in the cheddar and half the Parmesan.
If the macaroni is sticking together, pour a little boiling water over it, then drain and add to the sauce with the cauliflower. If the sauce is a bit thick, add a splash more milk. Tip into a large, shallow baking dish, sprinkle over the remaining Parmesan and top with the macadamias. Bake for 20 mins until cooked through and bubbling at the edges.