season the chicken breast pieces with salt and pepper. Cook til done in a large frying pan.
Put all the vegetables in a large mixing bowl, toss with olive oil, balsamic vinegar, salt and pepper. Sautee til onions are translucent and broccoli florets are soft, about 8-10 minutes
Butter the bread slices.
In a small bowl mix together 1 cup vegetables, 1 cup chicken pieces, 1 cup shredded cheese.
place one piece of bread butter side down in a medium high heated pan. Pile vegetable and chicken mixture. Add top bread slice butter side up and press firmly. Cover pan for 1 minute or so til butter stops sizzling and cheese has a chance to melt a little. Carefully flip the sandwich and cover again til butter stops sizzling.