Drop the cabbage in a pan of lightly salted boiling water. Bring back to the boil and simmer for 1 minute. Drain quickly and run under the cold tap.
Melt the butter and add the onion. Cook until soft without browning, then add the flour. Cook for a minute, stirring then gradually add the milk to make a white sauce. Bring to the boil and simmer gently for 10 minutes, stirring occasionally.
Add the cabbage, ham, half the parmesan, nutmeg and salt and pepper to taste. Heat through thoroughly then spread out in a gratin dish and sprinkle with the remaining parmesan.
Put under a very hot grill until the top is nicely speckled with brown.