Trim off and discard any thick, cauliflower and broccoli stalks. Break the heads into large florets, then cut off the thin stalks and reserve.
Bring a pan of salted water to the boil. Add the cauliflower and broccoli florets with the thin stalks and bring back to the boil. Boil for 4 minutes until they are just cooked. Drain and rinse under cold water and drain again. Spread out in the oven proof dish.
Preheat the oven to 200℃/180℃Fan/Gas 6.
Cheese sauce
Melt the butter in a saucepan, sprinkle in the flour, and cook, stirring with a hand whisk for 1 minute. Remove from the heat, and gradually blend in the milk. Bring to a boil, stirring all the time until thickened.
Simmer for 2 -3 minutes, remove from the heat then add both cheeses. Add the mustard, season with salt and pepper, and stir until combined.
Pour the sauce over the vegetables, and sprinkle with the cheddar cheese. Bake in the oven for 20 minutes. Serve hot.
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