Easy Chicken Caprese

Name: Easy Chicken Caprese
Ingredients: 2 pounds boneless skinless chicken breasts , cut in half lengthwise; 1/2 teaspoon salt , divided; 1/2 teaspoon freshly ground black pepper , divided; 1 tablespoon extra virgin olive oil; 2 tablespoons unsalted butter , divided; 1 medium yellow onion , finely chopped (about 3/4 cup); 3 cloves garlic , minced (about 1 tablespoon); 1/2 cup chicken stock; 1/3 cup balsamic vinegar; 1 cup cherry tomatoes; 1 tablespoon Italian seasoning; 8 ounces whole milk fresh mozzarella pearls , drained; 1/4 cup chopped fresh basil leaves