Preheat the oven to 180C.
Cut the broccoli into small florets, remove the leaves of the cauliflower, and break it into the same sized florets as the broccoli.
Bring a large pan of water to the boil. Add the broccoli and cauliflower florets and cook for about 5 minutes. Drain, then leave to steam dry.
Tip the broccoli and cauliflower into an oven proof dish, then make the cheese sauce.
Add the butter, flour, and milk straight into a saucepan. Cook over a medium to high heat, whisking continuously until the butter has melted and it starts to thicken.
Add the grated cheddar, parmesan and nutmeg and stir until the cheese has melted.
Turn the heat off and season with salt and pepper.
Pour the cheese sauce all over the broccoli and cauliflower, then scatter over the cheddar and parmesan.
Add the cubes of bread on top, then sprinkle with some chopped thyme (optional).
Bake it for 30-40 minutes, until it is bubbling and golden on top. Leave to stand for at least a few minutes before serving.