• Butter the oven proof dish.
• 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.