Preheat your oven to 375°F. Begin heating a large pot of well-salted water (about ¾ tablespoon kosher salt once boiling) for the pasta.
Heat a large skillet over medium-high heat. Add ½ lb. ground beef, ½ lb. ground sausage, and 1 small diced yellow onion. Cook for 4 minutes, breaking up the meat. Add 1 small diced green bell pepper and cook for 5 more minutes, until the meat is fully cooked through. Drain the grease.
Reduce heat to medium. Pour in ½ cup beef broth and scrape the bottom and sides of the skillet clean. Add 3 minced garlic cloves, 2 teaspoons Worcestershire sauce, 1 teaspoon hot sauce, 1 teaspoon dried oregano, 1 teaspoon dried basil, ½ teaspoon garlic salt, and 1 pinch red pepper flakes. Stir to combine and cook for 1 minute.
Add 32 oz. marinara sauce and 8 oz. pizza sauce. Stir to combine. Let the sauce simmer partially covered over medium heat while you cook the pasta.
Cook ¾ lb. penne pasta in the boiling salted water for 1 minute less than the package directions for al dente. Drain and set aside.
Reduce the sauce to low heat. Stir in ¼ cup grated Parmesan cheese. Add the drained penne and stir until everything is evenly combined.
Transfer the pasta mixture to a lightly greased 9x13-inch casserole dish (if your skillet is not oven-safe to 450°F). Spread it out evenly.
Scatter 2½ cups shredded whole milk mozzarella evenly over the top, then lay 25 pepperoni slices over the cheese.
Cover the dish with a lid or foil (if using foil, spray one side with non-stick cooking spray to prevent sticking). Bake covered at 375°F for 10 minutes.
Remove the cover and bake for 15 more minutes, until the cheese is fully melted and bubbly. For extra browning on top, broil at 450°F for about 1 minute — watch it closely and don't walk away.
Garnish with fresh parsley and extra grated Parmesan if desired. Serve hot.