01 - Pat the chicken breasts dry with paper towels. Season both sides generously with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts 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 chopped onion and sauté for 2 minutes until softened. Stir in minced garlic and sliced bell pepper; cook for another 2 minutes until fragrant.
04 - Add diced tomatoes with their juices, dried oregano, dried basil, and red pepper flakes if using. Stir well 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 sauce. Cover with lid and reduce heat to low. Simmer for 15-20 minutes until chicken is cooked through and reaches internal temperature of 165°F.
06 - Top each chicken breast with mozzarella cheese slices or shredded cheese. Cover skillet and cook for 2-3 minutes until cheese is completely melted and bubbly.
07 - Remove from heat. Garnish with fresh basil leaves and serve immediately while hot. Pairs excellently with pasta, rice, or crusty bread.