Preheat the oven to 220c First, mix the butter and flour to make a paste.
To make the sauce, place a knob of butter in a large pan and add the chopped leeks. Cook for a couple of minutes until the leeks start to soften and sweat down. Add the milk and bring to the boil.
Gently stir in a little of the flour and butter paste at a time until is has dissolved thoroughly. Bring to the boil and simmer for a minute then remove from the heat and allow to cool a little.
Blanch the cauliflower and broccoli florets in salted boiling water for around 3-4 minutes then drain into a colander. Or Microwave
Add the grated cheese to the sauce and mix in.
Place the blanched and dried cauliflower and broccoli florets and leaves into a big bowl and pour the cheese sauce over the top. Make sure you cover the cauliflower well and leave the bowl to stand for a couple of minutes so the cheese absorbs into all the nooks and crannies of the cauliflower.
Transfer to a large casserole dish (one you'd like to set out on the table) and sprinkle the breadcrumb and parmesan topping over the top. Bake in the oven for around 20-25 minutes or until golden and bubbly.
Serve straight from the dish you baked it in with some crusty bread on the side.