Step 1

Preheat the oven to 180ºC/350ºF/gas

Step 2

Put a large shallow ovenproof casserole pan on a medium heat with the butter. Trim, halve, wash and slice the leek and add to the pan. Stir regularly while you trim, slice and add the celery, then trim, chop and add the fennel, if using. Break the cauliflower into florets, then cut into 2cm-thick slices and finely slice the stalk and any nice leaves. Add the stalk and leaves to the pan and cook for 10 minutes, or until soft but not coloured, stirring regularly. Meanwhile, cook the pasta in a large pan of boiling salted water for 8 minutes (5 if using macaroni). Reserving a mugful of starchy cooking water, drain the pasta and pour into a 25cm x 35cm baking dish (or divide between smaller ones), along with the cauliflower florets. Stir the flour into the veg pan for 30 seconds, then gradually pour in the milk, stirring all the time, followed by 500ml of water. Simmer for 5 minutes, then stir in the mustard and season with sea salt and black pepper. Let the sauce blip away until thickened while you coarsely grate the cheese. Stir the cheese into the sauce, then blitz until smooth with a hand blender, or in batches in a food processor. Pour over the pasta and cauliflower florets then, if you need to loosen it, add a little reserved pasta water – the sauce will thicken up as it bakes. Blitz your stale bread into crumbs in the processor (or smash up in a clean tea towel). Peel and grate the garlic. Pick the herb leaves, if using, and mix it all together with 1 tablespoon of olive oil. Scatter the crumbs over the pasta, grating over more cheese, if you’ve got it, then bake for 30 minutes, or until golden and bubbling.