Preheat oven to 375°F (190°C). Unroll one ready-made pie crust and carefully press it into a 9-inch deep-dish pie plate. Crimp the edges as desired. If you wish to make a double-crust pie, reserve the second crust for the top; otherwise, set it aside or discard.

In a large skillet over medium-high heat, add ground beef and Italian sausage. Break up the meat with a spoon and cook until thoroughly browned, about 8-10 minutes. Drain off any excess fat from the skillet.

Add 1 tablespoon of olive oil to the same skillet. Reduce heat to medium. Add chopped yellow onion, green bell pepper, and red bell pepper. Sauté until softened, about 5-7 minutes. Stir in minced garlic and cook for another minute until fragrant.

Return the browned meat mixture to the skillet with the sautéed vegetables. Add quartered pepperoni slices, crushed tomatoes, tomato sauce, dried oregano, dried basil, and red pepper flakes (if using). Stir everything well to combine.

Season the filling with salt and black pepper to taste. Bring the mixture to a simmer, then reduce heat to low and let it cook for 10-15 minutes, allowing the flavors to meld and the sauce to thicken slightly.

Pour the hot meat and vegetable filling into the prepared pie crust in the deep-dish pie plate. Spread the filling evenly. Sprinkle 1.5 cups of shredded mozzarella cheese and 1/4 cup of grated Parmesan cheese over the top of the filling.

Place the pie plate on a baking sheet (to catch any potential overflow). Bake in the preheated oven for 35-45 minutes, or until the crust is golden brown and the cheese is melted, bubbly, and lightly golden.

Remove the Hearty Hamburger Beef Sausage & Pepperoni Pie from the oven and let it rest for 10-15 minutes before slicing and serving. This crucial resting time allows the filling to set, making it much easier to cut neat slices.