Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente. Drain the pasta and set it aside.

In a large oven-safe skillet over medium-high heat, cook the ground beef with the diced red onion and sliced mushrooms until the beef is browned and the vegetables are softened. Drain any excess fat from the skillet.

Stir the Italian seasoning, onion powder, garlic powder, and seasoned salt into the meat mixture. Mix well to evenly coat the beef and veggies.

Pour in the pizza sauce and pasta sauce, and add a few slices of pepperoni. Stir everything together and let it simmer for a few minutes to blend the flavors.

Add the cooked spaghetti to the skillet, tossing until all the noodles are coated in the sauce.

Top the spaghetti with the shredded mozzarella cheese, black olives, and the remaining pepperoni slices.

Transfer the skillet to an oven preheated to 350°F (175°C). Bake for 10 minutes until the cheese is melted. Then, switch the oven to broil for 2 to 3 minutes to achieve a bubbly, golden finish.

Serve hot directly from the skillet.