Put the macaroni in a large pan of boiling salted water and cook following the packet instructions. Drain and set aside.
Meanwhile part cook the broccoli in a pan of boiling salted water for 3 minutes then drain, refresh with cold water and set aside.
Pre-heat the oven to Gas 6, 200°C, fan 180°C and grease an ovenproof dish.
Melt the butter in a large pan over medium heat and fry the onion, for 5 minutes until soft. Add the garlic, mustard and flour and mix together. Add the milk a little at a time, whisking constantly to make a thick sauce. Add the Parmesan and half the Cheddar cheese and season to taste. Remove pan from the heat and stir the macaroni and broccoli into the sauce.
Pour the mixture into the ovenproof dish and arrange the tomato slices on the top. Sprinkle with the breadcrumbs and the remaining Cheddar cheese. Bake in the oven for 20 minutes, or until the top is brown and bubbling.
Make the recipe earlier in the day and bake when needed.
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