Preheat oven to 375℉.
In a 10-inch oven-safe skillet, heat the oil over medium-high heat. Once hot, add the sausage and cook until totally cooked through. Using a slotted spoon, transfer the cooked sausage to a plate and set aside.
Reduce the heat to medium. In the residual grease and oil, sauté the onions until translucent, about five minutes. Add in the garlic, oregano, red pepper flakes, and salt, and cook for an additional 2 minutes until the garlic is fragrant.
Add the sausage back into the pan. Pour in the marinara, beef broth, and pasta. Stir to combine. Add in ½ cup of the shredded parmesan and toss again to combine. Remove from the heat.
Tear the mozzarella into medium pieces and distribute evenly atop the surface of the pasta. Top with the remaining shredded parmesan cheese and the pepperoni slices. Cover the skillet with a lid (or tightly with foil) and bake for 20 minutes.
Remove the lid, or foil, and bake for an additional 15 until the pepperoni is crisp and the mozzarella is bubbly. Allow to cool for 10 minutes before serving. Garnish with fresh parsley.