Mix 3 tbsp of milk with the cornflour and mustard, set aside. Heat the rest of the milk with the garlic until just coming to boil. Remove from the heat, sprinkle in the crushed chillies and leave to infuse.
Get everything else ready. Coarsely grate both cheeses, keeping them separate. Mix a handful of the cheddar into the breadcrumbs with a grinding of pepper. Thickly slice the medium tomatoes and halve the cherry. Finely slice the spring onions. Heat oven to 190C/fan 170C/gas 5.
Bring a pan of water up to the boil, tip in the macaroni, give it a stir so it doesn’t stick, then cook for 6 mins, stirring occasionally. Stir in the spring onions and cook for another 2 mins. Meanwhile, make the sauce. Stir the cornflour mix into the warm milk. Return the pan to the heat, then bring to the boil, stirring, until thickened and smooth. Remove from the heat and stir in the Parmesan, most of the remaining cheddar and some pepper to taste. Stir in the buttermilk.
Tip the macaroni into a colander, drain, then hold under a very hot tap to keep it all separate. Drain well, then stir into the sauce. Pour into an ovenproof dish, about 30 x 20 x 5.5cm deep. Lay the tomatoes over the top then scatter over the cheesy breadcrumbs, the rest of the cheese and a grating of pepper. Bake for about 15 mins until starting to bubble around the edges. Grill for about 5 mins until the top is crisp and well browned. Let sit for a few mins to settle before serving.
