In a 12” skillet over medium heat, sauté onion in oil until softened, about 3-5 minutes. Add sausage, browning the edges (I use fully-cooked sausage). Add pepperoni, diced tomatoes, tomato sauce, pasta, chicken broth and Italian seasoning.

Bring to a boil, cover and simmer until most of the liquid is absorbed, about 20 minutes. Top with cheese and extra pepperoni. Place the lid over the skillet until the cheese melts.

Sprinkle with red pepper flakes. Serve immediately.