Sprinkle courgettes with salt and leave to rest for 15 to 30 minutes to extract some of the liquid. Squeeze dry with a clean towel.
Preheat oven to 180 C / Gas 4.
Saute onions in oil and half of the butter until transparent. Stir in courgettes. Cook, turning from time to time, until tender.
Combine courgette mixture with the cheese and cream. Beat eggs, fold into mixture. Line a baking dish. Spoon mixture into dish, and stand dish in a larger dish or roasting tin with water at the bottom.
Bake in the preheated oven for 35 to 40 minutes. To test for readiness plunge knife into centre and if it is almost clean when removed, the mould is ready. Leave to set for about 20 minutes.
For garnish, heat the remaining 3 tablespoons butter, stir in breadcrumbs and cook till slightly brown. When cool, stir in parsley and seasoning. Turn out mould and sprinkle with garnish.