Remove the outer leaves and wash well both the cauliflower and the leaves. Put not more than 1 inch (2.5 cm) of water in a saucepan just large enough to take the cauliflower; add a little salt. Chop the leaves into small pieces and either leave the cauliflower whole or cut in quarters; place the cauliflower on top of the green leaves in the saucepan, cover and simmer until the cauliflower is cooked, 15 minutes approx. Test by piercing the stalk with a knife: there should be just a little resistance. Remove the cauliflower and leaves to an ovenproof serving dish.
Meanwhile make the Mornay Sauce. Put the cold milk into a saucepan with a slice of onion, 3–4 slices of carrot, 6 peppercorns and a sprig of thyme or parsley. Bring to the boil, simmer for 3–4 minutes, remove from the heat and leave to infuse for 10 minutes. Strain out the vegetables, bring the milk back to the boil and thicken with roux to a light coating consistency. Add 4 ozs (110 g/1 cup) grated cheese and a little mustard. Season with salt and freshly ground pepper, taste and correct the seasoning if necessary. Spoon the sauce over the cauliflower and sprinkle with more grated cheese. The dish may be prepared ahead to this point.
Put into a hot oven, 230°C/450°F/regulo 8, or under the grill to brown. If the Cauliflower Cheese is allowed to get completely cold, it will take 20–25 minutes to reheat in a moderate oven, 180°C/350°F/regulo 4.
Serve sprinkled with chopped parsley.