Using a fork, prick small holes into the frozen pie crust. Place in oven for 10 minutes. When done, remove from oven and let cool.
Meanwhile, heat oil in a large frying pan. Saute onion for 5-7 minutes, until translucent.
Add mushrooms to pan. On medium-low heat, saute mushrooms, stirring occasionally until mushrooms have reduced in size and darkened in color. This will take between 10 and 20 minutes.
Add garlic to pan and saute for an additional minute or two.
Set mushroom mixture aside and let cool.
In a small bowl, combine heavy cream, egg, egg whites, flour, muenster cheese, salt, and pepper. Mix well.
Set the pie crust on top of a baking sheet in case of leaks.
Pour mushroom mixture into pie crust.
Pour cheese mixture over the mushrooms. Sprinkle with thyme.
Bake at 350 F for 40 minutes or until golden brown and top is set.
Remove from oven and let cool slightly before slicing so the inside has a chance to firm up.