1. Put the cauliflower and leek into a large saucepan and cover with stock. Bring to the boil then simmer for 10–12 minutes or until the cauliflower is just tender. Drain well. Reserve the cooking liquid to use in soups and stews.
2. Meanwhile, put the reduced-fat spread, flour and milk in a small saucepan. Set over a medium heat and whisk constantly until the mixture comes to the boil. Reduce the heat, continue whisking until the sauce is thickened and smooth. Stir in all but a teaspoon of the cheese and season with pepper.
3. Divide the cauliflower mixture between two small, deep, ovenproof dishes. Spoon the sauce over and sprinkle with the reserved cheese.
4. Grate the bread into coarse breadcrumbs over the top. If freezing for future use, freeze now, then defrost overnight in the fridge before using.
5. If eating immediately, grill until golden. Alternatively, keep chilled for up to three days and then bake at 170°C/140°C fan/gas mark 3 for 40 minutes until piping hot.