Peel the zucchini and cut them into two-inch rounds. Peel the onions and cut them into thin rings.
Cook the zucchini and onions in the butter until light golden. Do not brown and do not let the zucchini become soft.
Spoon the zucchini and onions into a shallow casserole. Combine the eggs, cream, nutmeg and half of the cheese. Pour the mixture over the vegetables. Sprinkle with salt and pepper and the remaining cheese.
Set the casserole in a baking dish, pour boiling water around it, and bake for thirty-five to forty minutes, until custard is set.