Cook the two types of cabbage in boiling water for 5 minutes until just tender. Drain and place in a large mixing bowl and toss with the nutmeg and peanuts.
Heat the butter in a nonstick saucepan until melted. Remove from the heat, add the flour and mix to a paste. Return to the heat and cook for a few seconds. Remove from the heat and add the milk a little at a time, stirring well between each addition. Return to the heat and bring to the boil, stirring continuously until boiled and thickened.
Remove the pan from the heat and add 75 g (3 oz) of the grated Cheddar and the mustard. Mix well, then pour over the cabbage and mix it in.
Transfer the mixture to a gratin dish, or 4 individual gratin dishes. Toss the remaining cheese with the parsley and breadcrumbs, then sprinkle over the top and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until golden and bubbling.