Cook the pasta according to the directions on the box, and drain.
Meanwhile, in a Dutch oven over medium heat, combine the butter, sirloin, onion, and garlic.
Cook and stir until meat is browned. Drain any liquid from the pan.
Add the pizza sauce, garlic salt, and Italian seasoning.
Remove from heat, and set aside.
In a large bowl, stir together eggs, ricotta cheese, milk, and Parmesan cheese.
Add the pasta to the egg and cheese mixture, and stir gently to coat.
Transfer the pasta to a lightly greased 3-quart baking dish or 9x13-inch baking dish.
Spoon the beef and sauce mixture evenly over the pasta.
Scatter half the pepperoni slices over the beef mixture.
Sprinkle evenly with mozzarella cheese.
Scatter remaining pepperoni slices over the mozzarella cheese.
Cover with aluminum foil and bake at 350 degrees for 20 minutes.
Remove foil and continue baking for another 20-30 minutes, or until cheese is melted and golden.
Cut and serve.