1. Preheat your oven to 375°F (190°C).
2. In a large skillet over medium heat, cook the ground beef and sausage until browned, breaking it apart with a spoon. This should take about 7-8 minutes.
3. Add the chopped onion and garlic to the skillet, cooking until the onion is translucent, approximately 5 minutes.
4. Stir in the pizza sauce, oregano, basil, and red pepper flakes (if using). Simmer for another 5 minutes, then remove from heat.
5. Roll out one pie crust and fit it into a 9-inch pie dish, allowing excess to hang over the edge.
6. Spoon the meat mixture into the pie crust, spreading it evenly. Layer the pepperoni slices on top, followed by the shredded mozzarella cheese.
7. Cover with the second pie crust, sealing the edges with a fork or by crimping with your fingers. Cut a few slits in the top crust to allow steam to escape.
8. Brush the top crust with the beaten egg to ensure a golden, flaky finish.
9. Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown. Let it cool for at least 10 minutes before slicing.

One of my favorite tips is to let the pie rest before serving; it allows the flavors to meld beautifully, reminiscent of the patience my grandmother taught me in the kitchen.