Heat oven to 180C and line a 11” x 7” glass baking dish or similar with parchment paper.
In a medium to large saucepan heat the olive oil and sauté the button mushrooms until tender.
Spread the mushrooms and caramelized onions over the baking dish.
In a mixing bowl whisk together the eggs and cream. Add the cheese and the chopped parsley. Season and sir.
Pour this mix over the mushrooms and onions in the baking tray and put in oven for 18 minutes. You can allow to stand for 5 minutes before serving hot, or allow to come to room temperature and slice up into nice little squares.