Make a batch of pie crust. (You can make this quiche in a pie dish or a sheet pan. The pie dish will require longer baking. A full sheet pan will require a double batch of pie dough, or double crust recipe.) Add a handful of chopped fresh rosemary if you want! Place crust in pan and chill while you make the filling.
FILLING ½ stick of butter
2 medium onions, chopped very small
2 medium sweet potatoes, peeled and chopped very small
3 cups chopped mushrooms
Sautee vegetables in butter until tender Drain excess liquid, and place filling in chilled pie crust.
6 cups half & half
3 Tablespoons soy sauce
Whisk together and pour evenly over filling. Sprinkle plenty of cheddar cheese over the top. Bake in a preheated 450 degree oven for 15 minutes, then turn to 325 degrees for a further 15-20 minutes, or until golden brown.