8 boneless skinless chicken breast halves, well trimmed
2 tablespoons balsamic vinegar or red wine vinegar
1 tablespoon olive oil
2 garlic cloves, minced
2 teaspoons minced fresh rosemary
2 red onions, cut into 1/2-inch-thick slices
8 large French bread rolls (about 4 inches long), cut in half
1/2 cup chopped drained oil-packed sun-dried tomatoes
2 bunches arugula
4 ounces light soft goat cheese