Preheat oven to 175°C.

Lightly coat a large ovenproof sauté pan with extra virgin olive oil, and place over medium-high heat. Sauté mushrooms for 7 to 10 minutes, until caramelized with slightly crispy edges. Add shallots, parsley, and sage. Continue cooking for 3 to 5 minutes or until shallots are limp and golden. Transfer to a large mixing bowl and let cool.

Add Swiss cheese, Parmesan, eggs, and cream to the cooled mushroom mixture. Mix thoroughly and season with salt and pepper.

Bake at 175°C for 20 to 30 minutes or until the centre is just barely set. Frittata will continue to cook after removing from the oven, so resist the urge to overbake.

Rest in pan for 5 minutes before slicing and serving.