Preheat oven to 350 degrees Fahrenheit.

In a bowl, combine the crumbs, butter and parmesan and press into an eight-inch springform pan. Bake for 10 minutes. Turn the oven down to 300 degrees Fahrenheit.

In a pan, sauté the onion in the two tablespoons of butter, add the mushrooms and cook until the mushrooms give up their liquid and are dry and lightly browned. Add the spinach and continue cooking for a few minutes or until the spinach is wilted.

Mix the cheeses, eggs, cream, seasonings, and additional parmesan until smooth. Fold in the mushroom-onion mixture. Pour onto the prepared crust and bake for one and a half hours in the 300-degree-Fahrenheit oven.