01 - Pat the chicken breasts dry with paper towels. Generously season both sides with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Place chicken breasts in the hot pan and sear for 3-4 minutes per side until golden brown. Remove chicken from skillet and set aside on a plate.
03 - In the same skillet, add the chopped onion. Sauté for 2 minutes until softened and translucent. Add minced garlic and sliced bell pepper; cook for another 2 minutes, stirring frequently to prevent burning.
04 - Pour in the diced tomatoes along with their juices. Add dried oregano, dried basil, and red pepper flakes if using. Stir well to combine and let the mixture simmer for 5 minutes to allow flavors to meld.
05 - Return the chicken breasts to the skillet, nestling them into the tomato sauce. Cover with a lid and reduce heat to low. Simmer for 15-20 minutes, or until the chicken reaches an internal temperature of 165°F.
06 - Top each chicken breast with mozzarella cheese slices or shredded cheese. Cover the skillet again and cook for 2-3 minutes until the cheese is completely melted and bubbly.
07 - Remove from heat. Garnish generously with fresh basil leaves. Serve immediately while hot, accompanied by pasta, rice, or crusty bread to soak up the sauce.