- Preheat the oven to 180°C/gas 4. Add the cauliflower to a saucepan of boiling water and, after 3 minutes, add the broccoli. Cook a further 6 minutes until almost cooked but still firm. Drain and set aside.
- Meanwhile, add the oil to a saucepan and cook the onion, soften for 3-4 minutes but don’t brown it. Sprinkle the flour over the onions, add the mustard and gently stir for 1 minute to coat all the onions.
- Gradually pour in 100ml of the milk, mixing to ensure there are no lumps. As it starts to thicken, gradually stir in the rest of the milk until it becomes a sauce. Add the 25g of Cheddar and all the Parmesan, stirring until the cheeses have melted.
- Arrange the broccoli and cauliflower in an ovenproof dish, pour the sauce over and top with the remaining Cheddar. Bake for 20-25 minutes, until the top has browned.
- If you have any left over, blend with some vegetable or chicken stock for a great soup.
- This dish is delicious with 1 tsp curry paste added to the sauce with the mustard in step 2.
- You can assemble this in advance and leave it in the fridge then just pop it in the oven when needed. If baking straight from the fridge, add an extra 5-10 minutes to the cooking time.
- Freezing instructions: Suitable for freezing once cooked. Defrost in the fridge or microwave and reheat until piping hot throughout.