Step 1: In a large skillet, brown the ground beef and Italian sausage over medium-high heat. Break up the meat as it cooks. Once fully browned, drain off any excess grease and set the meat aside.
Step 2: In the same skillet, add the chopped onion and bell pepper. Sauté until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Step 3: Return the browned meat to the skillet. Pour in the marinara sauce, diced tomatoes, and Italian seasoning. Bring the mixture to a gentle simmer, then reduce heat to low, cover, and let it cook for at least 15-20 minutes, allowing the flavors to meld. Season with salt and black pepper to taste.
Step 4: Preheat your oven to 375°F (190°C). Lightly grease a 9-inch deep-dish pie plate or springform pan.
Step 5: Carefully unroll one pie crust or press one portion of pizza dough into the bottom and up the sides of your prepared pan. Trim any excess dough, leaving about a 1/2-inch overhang.
Step 6: Sprinkle 1.5 cups of the shredded mozzarella cheese evenly over the bottom of the crust.
Step 7: Spoon the meat and sauce mixture over the mozzarella layer, spreading it evenly.
Step 8: Top with the sliced pepperoni, followed by the remaining 1.5 cups of mozzarella cheese. If using, sprinkle grated Parmesan cheese over the top.
Step 9: Place the second pie crust or remaining pizza dough over the top of the pie. Crimp the edges together with the bottom crust to seal, or fold and tuck if using pizza dough in a springform pan. Cut a few slits in the top crust to allow steam to escape. If desired, brush the top crust with the egg wash for a golden finish.
Step 10: Bake for 35-45 minutes, or until the crust is golden brown and the filling is bubbly. If the crust starts to brown too quickly, loosely tent it with aluminum foil.
Step 11: Let the pie rest for at least 10-15 minutes before slicing and serving. This allows the filling to set, making for cleaner slices.