Preheat oven to 400°F. Line a baking sheet with foil. Place balsamic vinegar and oil on baking sheet, add tomatoes and toss to coat. Place to one side of the baking sheet and sprinkle with 1/4 teaspoon garlic salt.
Season chicken with remaining garlic salt and Italian seasoning. Place on baking sheet next to tomatoes. Roast 30 minutes or until chicken is cooked through.
Top chicken with mozzarella and cook 5 minutes more to melt cheese. Serve chicken, tomatoes and cooking juices over cooked polenta slices, if desired. Top with walnuts and basil.