Preheat the oven to 200°C/gas mark 6. Bring a large pan of water to the boil, add the macaroni and cook for 5 minutes, then add the broccoli and cook for a further 3 minutes. Drain, then tip into a 2-litre baking dish.
CHILDREN: Make a paste with the cornflour and 2 tbsp milk. Grate the cheese.
Pour the rest of the milk into a pan and bring to the boil. Add the paste and whisk continuously with a balloon whisk until you have a smooth, velvety sauce.
Stir in the spring onions, mustard and two-thirds of the cheese, then pour the sauce over the macaroni and broccoli.
CHILDREN: Mix the breadcrumbs with the thyme and the remaining cheese. Arrange the tomato slices over the top of the dish, slightly overlapping as you go, then scatter the herby breadcrumbs over the top.
Bake for 20-25 minutes until the top is golden and crusty and the filling is bubbling. Allow to stand for 5 minutes, season with freshly ground black pepper and serve.