Preheat oven to 375°F. Grease a 9-inch pie dish with cooking spray or butter.
In a medium skillet over medium heat, add 1 tablespoon butter and mushrooms. Sautee until softened and browned, about 5 minutes; transfer to the prepared pie dish.
In the same skillet, heat the remaining 2 tablespoons butter, add the zucchini, onions and tomatoes and saute until softened and lightly browned, about 5 minutes. Add to the mushrooms on the prepared pie dish. Sprinkle with ½ cup of gruyere cheese.
In a large bowl, beat the eggs with the milk, salt, pepper and nutmeg until well blended. Pour the egg mixture over the top of the vegetables. Sprinkle with the remaining ½ cup of gruyere cheese.
Bake in the preheated oven until a knife inserted in the center comes out clean, 30-35 minutes. Let stand 10 minutes before cutting.
Cut into 6 wedges, garnish with parsley and serve warm or at room temperature.