01 - Pat the chicken breasts dry with paper towels and season both sides generously with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Place chicken breasts in the pan and sear for 3-4 minutes per side until golden brown. Remove from skillet and set aside on a plate.
03 - Add the chopped onion to the same skillet and sauté for 2 minutes until softened and translucent. Stir in the minced garlic and sliced bell pepper; cook for another 2 minutes until fragrant.
04 - Pour in the diced tomatoes, 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 seared chicken breasts to the skillet, nestling them into the sauce. Cover with a lid and reduce heat to low. Simmer for 15-20 minutes until chicken is cooked through and 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 and garnish with fresh basil leaves. Serve immediately while hot, accompanied by pasta, rice, or crusty bread.