Preheat the oven to 200°C/180°C fan/Gas 6. Lightly butter a medium, shallow baking dish.
Cook the macaroni in well-salted boiling water for about 10 minutes, until just tender with a slight bite. Drain well.
Melt the butter in a large saucepan over medium heat. Stir in the flour and cook for 1 minute, stirring constantly, until pale and sandy.
Gradually whisk in the hot milk, a splash at a time, until smooth. Keep stirring over medium heat for about 5 minutes, until the sauce thickens and coats the back of a spoon.
Take off the heat. Stir in most of the Cheddar and Parmesan (keep a handful of each back for the top) and the Dijon mustard. Season well with salt and pepper.
Fold the drained macaroni into the sauce until fully coated.
Pour into the prepared baking dish. Scatter over the reserved cheese, then press the cherry tomatoes, cut-side up, across the top.
Bake for 20 to 25 minutes until bubbling and golden, and the tomatoes have softened. Rest for 5 minutes, then scatter over the parsley to serve.