Heat the olive oil in a frying pan and saute the chopped onions over low heat for 5-7 minutes, then add the garlic and cook briefly for 30 seconds while stirring. Add the mushrooms with a pinch of pepper and chopped rosemary and brown over medium heat until golden. Add salt when mushrooms are close to done.

Sprinkle the mushrooms with flour and stir to coat. Deglaze the pan with the Marsala wine and let it bubble away for about 5 minutes until it's reduced by a half. Then add the stock and cook for 5 minutes longer. At the end stir in the double/heavy cream and let it warm through. Taste for seasoning and add more salt if needed, then take off the heat. Sprinkle with fresh parsley before serving if using.