Preheat oven to 425 degrees F. On a lightly floured surface, roll Rich Tart Pastry dough from center to edges into a 12-inch square. Transfer pastry to a 9-inch square tart pan with removable bottom. Ease pastry into pan, being careful not to stretch pastry. Press pastry edges against pan edges. Trim excess pastry. Line pastry with a double thickness of heavy foil; press down firmly but carefully. Place tart pan on baking sheet.
Bake for 8 minutes. Remove foil. Bake for 8 to 10 minutes more or until pastry is golden. Remove from oven. Reduce oven temperature to 325 degrees F.
Meanwhile, in a large skillet, cook pancetta or bacon until crisp. Remove pancetta or bacon to paper towels. Reserve 1 tablespoon drippings in skillet. Add mushrooms, onion, and garlic to drippings in skillet; cook and stir over medium heat until vegetables are tender. Remove from heat.
For filling, in a medium bowl, whisk together eggs and half-and-half or light cream. Stir in oregano or thyme and pepper. Sprinkle mushroom mixture, pancetta or bacon, and cheese into prebaked crust. Carefully pour egg mixture into crust (pan will be full).
Bake for 25 to 30 minutes or until a knife inserted near the center comes out clean. Let cool in pan on a wire rack for 15 minutes. Remove side of pan. Cut into squares and serve warm. If desired, garnish with oregano or thyme sprigs. Makes 12 servings.
Rich Tart Pastry
In a medium bowl, stir together all-purpose flour and salt. Using a pastry blender, cut in butter until pieces are pea-size. In a small bowl, whisk together egg yolk and 1 tablespoon ice water. Gradually stir egg yolk mixture into flour mixture. Add ice water, 1 tablespoon at a time, tossing gently until all of the dough is moistened. Using your fingers, gently knead the dough just until a ball forms. Cover dough with plastic wrap. Chill about 1 hour or until dough is easy to handle.