1. Fill your kettle and boil it. Slice the leaves off the cauliflower and set them aside. Break the cauliflower into bite-sized florets and place them in a colander or steaming basket. Roughly chop any large chunks of stalk and add them to the florets. Tuck the bay leaf in with the cauliflower. Fill a pan or steamer with hot water from the kettle, set the colander with the cauliflower over the pan, cover with a lid (or baking tray if you don’t have a lid) and put the pan on a medium-high heat. Steam the cauliflower for 10-12 mins till it’s tender when pierced with a skewer.
2. Meanwhile, finely slice the cauliflower leaves. Trim the roots and top 3cm off the leek. Halve it and rinse out any grit, then finely slice. Set aside.
3. Place a deep frying pan or wok pan on a medium heat and add the gnocchi. Dry fry the gnocchi, stirring every so often, for 3-4 mins till lightly browned. Tip into a heatproof dish and set aside. Wipe the pan clean.
4. Add 2 tbsp olive oil to the empty pan and add the shredded cauliflower leaves and the leeks. Season with a pinch of salt and pepper. Fry, stirring often, for 8 mins till the veg have softened and picked up a little colour (but not too much, if they begin to brown, turn the heat down a little).
5. Stir 2 tbsp flour into the veg and cook, stirring for 2 mins. Add ½ tbsp Dijon mustard and stir to mix. Add a splash of milk, around 50-75ml, and stir to combine with the veg and flour. When the milk is smoothly incorporated, add a splash more and stir again. Carry on till you’ve added 500ml milk and have a smooth sauce.
6. Bring the sauce to a boil, stirring, then turn the heat down and simmer for 5 mins. Stir often till the sauce thickens. Set your grill to high.
7. When the cauliflower is tender, take it off the heat and add it to the dish with the gnocchi. Discard the bay leaf. When the white sauce has thickened a little, take it off the heat and grate in a quarter of the nutmeg. Coarsely grate the cheddar and add three quarters to the pan. Stir to mix, taste and add more nutmeg, salt or pepper if needed.
8. Pour the cheese sauce over the gnocchi and cauliflower to cover them. Scatter over the breadcrumbs, and the last of the cheddar. Slide under the grill for 8-10 mins, till golden and bubbling. Let it cool for a few mins before serving.