Heat olive oil in a large oven-safe skillet over medium heat.
Add sliced sausage and cook until browned on all sides. Remove and set aside.
In the same skillet, add onion and bell peppers.
Cook until softened, about 5 minutes.
Stir in minced garlic, Italian seasoning, and red pepper flakes; cook for 1 minute.
Return sausage to the skillet.
Stir in marinara sauce and season with salt and pepper.
Mix everything together and remove from heat.
Sprinkle shredded mozzarella evenly over the top.
Place skillet under a broiler or in a 400°F (200°C) oven for 8–10 minutes until cheese is melted and bubbly.
Garnish with fresh parsley.
Serve hot as is or over pasta, rice, or crusty bread.
