Brown the ground beef: Heat a large oven-safe skillet over medium-high heat and add the ground beef, breaking it into small crumbles with a spatula until no pink remains, about 6 minutes.

Drain excess fat: Tilt the skillet and spoon off or drain any excess grease so the sauce doesn't turn greasy later.

Cook the vegetables: Add the diced onion and bell pepper to the skillet, stirring until softened and starting to turn translucent at the edges, about 4 minutes.

Add garlic and seasoning: Stir in the minced garlic, Italian seasoning, and salt, cooking until fragrant, about 1 minute.

Simmer with marinara: Pour in the marinara sauce and let it simmer with the meat mixture until it thickens slightly and coats everything evenly, about 5 minutes.

Preheat the oven: While the sauce simmers, preheat your oven to 400°F so it's ready for baking.

Top with mozzarella: Sprinkle the shredded mozzarella cheese evenly over the meat mixture in the skillet.

Layer the pepperoni: Arrange the pepperoni slices over the cheese so each piece has space to crisp as it bakes.

Bake until bubbly: Transfer the skillet to the oven and bake until the cheese is fully melted and the edges turn golden and bubbly, about 10 minutes.

Finish and serve: Remove from the oven, sprinkle with parmesan cheese and red pepper flakes if using, and divide into bowls.