Delicious crusted whole cauliflower on a bed of creamy leeks.
Heat the oven to 200°C/180°C fan. Remove the tough outer leaves of the cauliflower, leaving any small, tender leaves intact. Trim the base so the cauliflower sits flat, then put into large saucepan and fill with water. Bring to boil and boil for 20 mins or until tender, drain and pat dry.
Sit cauliflower upright in a roasting tin. Rub all over with the 50g butter, season generously with salt and pepper, then cover with foil. Roast for 10 minutes, remove the foil, baste with the buttery juices, then return to the oven, uncovered, for 20-30 minutes more until the cauliflower is soft, including the stalk and caramelised on the outside. Leave to cool.
For the cheese crumb coating, whizz the breadcrumbs, cheese and thyme in a food processor, then mix in the melted butter to combine and season. Use your hands to work into a pliable coating, then press on top of the cooled cauliflower, covering it from the top as far down as it will go. Chill for 40 minutes to firm the crust, you can do this a day or two ahead.
For the cheese sauce, pour milk into a medium saucepan and add flour and butter. Turn heat on to medium and whisk, keep whisking as the butter melts and as the mixture comes to the boil. When the mix is glossy and smooth add the cheese. Stir to combine.
Meanwhile, heat the oven to 200°C/180°C fan, then bake the chilled cauliflower for 20-30 minutes until golden.
For the creamed leeks, melt the 50g butter in a large pan, add the sliced leeks and cook gently over a low heat until softened. Add the cream, turn up the heat and cook until thickened. Stir in the mustard and lemon zest, then season to taste with salt and pepper. Transfer to a warmed serving dish, forming a base for the cauliflower to sit on.
Carefully transfer the cauliflower onto the platter, sitting it on the bed of creamed leeks. Drizzle a small amount of the cheese sauce over cauliflower and scatter with fresh thyme (if using). The remaining cheese sauce can be served in small jug alongside.