Preheat the oven to 220°C/200°C Fan/Gas 7.
Bring a large saucepan of water to the boil. Add the cauliflower florets and cook for 3-4 minutes – you don’t want them too soft and they should still have a bite to them. Drain and refresh the cauliflower under cold water.
To make the cheese sauce, melt the butter in a saucepan. Add the flour and stir thoroughly for a couple of minutes to cook the flour – – this is important, as the flour can taste raw if not toasted in this way first. Gradually add the milk, about 100ml at a time, stirring constantly between each addition, until it is all incorporated and you have a smooth if runny sauce.
Turn up the heat slightly and continue to stir the sauce until it comes to the boil and thickens. Season with salt and pepper. Add 75g of the cheese and stir until it has melted into the sauce.
Put the cauliflower in an ovenproof dish and pour the sauce over it. Arrange the slices of tomato on top. Mix the remaining cheese with the breadcrumbs, if using, and sprinkle it over the tomatoes. Put the dish in the oven and bake for 15-20 minutes until the cheese is golden brown and bubbling.