1. Marinate mushrooms in balsamic vinegar for at least 24 hrs under refrigeration, stirring occasionally. Discard any excess marinade when ready to assumble sandwiches.
2. Slice peppers into 1/4-in wide strips.
3. Slice tomatoes into 1/4-in thick slices.
4. Trim any fat or rib meat from the chicken breasts & grill till done.
5. To assemble sandwich: Slice 1 roll in half lengthwise. Spread 1 oz of pesto on the bottom of the roll, then crumble 1 oz of feta on top of pesto, covering the entire surface.
6. Place a chicken breast on top of the feta, then 1/2 oz of mushrooms on the chicken, 1 oz of roasted red pepper strips on top of the mushrooms & finish w/3 tomato slices & the top of the roll.
7. Insert 2 toothpicks, slice on the diagonal & serve.