Break broccoli and cauliflower into large florets, finely chop the cauliflower leaves and add them all to a large ovenproof dish. Drizzle over a little olive oil, season generously with salt + pepper and roast for 20 minutes.
Place a saucepan over medium heat, add the butter and melt. Heat until it just begins to bubble.
Add the flour to the melted and combine using a whisk, allow to cook out for 1 to 2 minutes.
Add the finely chopped rosemary and thyme, and cook for another minute.
Gradually add the milk, whisking continuously, until you have a thick sauce. This will take between 10 ad 15 minutes. Stir in the mustard powder if using.
Remove from the sauce from the heat and add ⅔ of each cheese, stir a briefly and let the cheese melt - 5 - 6 minutes*
Tip the cheese sauce over the cauliflower & broccoli, top with the remaining cheese and a few cracks of black pepper, and return to the oven for 15 - 20 minutes, or until the top is golden, bubbling and crunchy.
* If the cheese needs a little more encouragement to fully melt, pop back over low heat and stir continuously until the sauce is velvety and smooth.