6 skinless, boneless chicken breast halves

ground black pepper to taste

1 teaspoon garlic salt

2 tablespoons olive oil

1 onion, thinly sliced

1/2 cup balsamic vinegar

1 (14.5 ounce) can diced tomatoes

1 teaspoon dried basil

1 teaspoon dried oregano

1 teaspoon dried rosemary

1/2 teaspoon dried thyme