For pie crust, use a basic pie crust recipe. To bake, roll out crust and fit into pan. Fit another piepan inside with a weight or line pan with aluminum foil and fill with uncooked rice or beans. Bake at 400 for 7 minutes. Remove 2nd pie pan or beans & rice. Prick bottom of crust and bake 5 minutes more.
For filling: melt butter in saucepan. Add mushrooms, onion and salt and saute. Cook spinach until tender. Drain well. Combine spinach & saute. mix & spread over bottom of pie pan. Grate cheese & spread evenly on top of spinach mix.
Custard*: Melt butter. Beat together above ingredients, pour over spinach mixture. Bake pie for about 40 minutes at 375 or until knife comes out clean.