1 pound boneless skinless chicken breasts
3 tablespoons olive oil, divided
2 tablespoons butter, divided
1/2 cup diced onion
1 tablespoon balsamic vinegar
4 garlic cloves grated
14.5 ounces canned diced tomatoes
1/4 cup chopped fresh basil, plus more for garnish
1/3 cup shredded mozzarella cheese (or thin mozzarella slices)
Sprinkle salt and pepper on both sides of chicken breasts.
Heat skillet over medium heat. Add 2 tablespoons olive oil and 1 tablespoon butter. When oil ripples and butter has melted, add chicken breasts and cook for 3 minutes on each side. Remove chicken from pan and set aside.
Add remaining 1 tablespoon olive oil and 1 tablespoon butter. When butter has melted, add onion and saute for 5 minutes. Add balsamic vinegar and grated garlic; cook, stirring for 1 minute.
Stir in diced tomatoes and fresh basil; bring to a simmer. Continue to simmer for 10 minutes, stirring occasionally. Add additional salt and pepper to taste.
Reduce heat to low. Tuck cooked chicken breasts back into sauce, turn breasts to coat with sauce. Top each chicken breast with mozzarella. Cover and cook until cheese melts, or place under broiler (if skillet is oven-safe) to melt cheese.
Garnish with additional fresh basil.