Cook ground sirloin in medium frying pan with diced onion, onion salt, garlic and pepper until no longer pink. Stir in 1 cup mini pepperoni.
Boil spaghetti noodles until al dente. Drain, reserving 3 tablespoons pasta water.
In a large bowl stir together eggs, Parmesan cheese and cooked spaghetti noodles.
Spread thin layer of marinara in bottom of greased 13x9 pan. Top with spaghetti noodle mixture.
In a bowl, stir together marinara and tomato sauce with reserved pasta water. Spread over noodles. Sprinkle cooked beef and pepperoni mixture onto sauce.
Layer with mozzarella cheese and top with mini pepperoni.
Cover with foil and bake 45 minutes. Uncover and bake until cheese is golden and pepperoni is crispy around the edges, 15-20 minutes.
Remove from oven and let rest 15 minutes. Slice. Serve. Refrigerate leftovers.