Let pastry shell stand at room temperature for 10 minutes. Line unpricked pastry shell with a double thickness of heavy-duty foil. Bake at 400° for 4 minutes. Remove foil; bake 4 minutes longer. Cool on a wire rack. Reduce heat to 350°.
In a large skillet, saute the onion, mushrooms and squash in butter until tender. Add spinach and garlic; cook 1 minute longer. Spoon into crust; top with cheese.
In a large bowl, whisk the eggs, cream, salt, rosemary and pepper until blended; pour over cheese.
Cover edges of crust loosely with foil. Bake for 40-45 minutes or until a knife inserted in the center comes out clean. Let stand for 10 minutes before cutting.
