Heat oven to 400ºF.
Roll out pastry sheet on floured surface to 1/8-inch thickness. Dock the dough at 1-inch intervals. Trim pastry to 15x12-inch rectangle.
Press the dough into half hotel pan that has been sprayed with pan release. Line pastry with aluminum foil and fill with pie weights, uncooked rice or beans. Bake for 10 minutes. Remove foil and weights; bake for additional 5 minutes on the bottom rack. Set aside while preparing filling ingredients.
Reduce oven temperature to 375ºF.
Melt half the butter in heavy saucepan. Add leeks; cover. Sweat on low heat until leeks are tender. Remove from heat.
Melt remainder of butter in another heavy saucepan. Add mushrooms; cover. Sweat on low heat until mushrooms have released their juices. Remove cover; increase heat to medium. Continue cooking until all liquid has evaporated and mushrooms have started to sauté. Stir mushrooms into leek mixture.
Whisk eggs, Cream Sauce Base, milk, kosher salt and pepper in large bowl.
Place Swiss cheese strips onto bottom of pre-baked pastry; top with mushroom leek mixture and egg mixture.
Bake for 45 to 55 minutes on middle rack until knife inserted in center comes out clean. Let stand 15 minutes (or refrigerate). Trim edges and cut 8x5 (1 1/2-inch squares).