Preheat oven to 400°F (200°C) (after marinating and ready to roast)
Wash peppers and slice off the tops. Remove and discard seeds.
Slice the peppers into chunks or slices (either is fine)
Peel and slice the onion into wedges.
Place pepper pieces and onion wedges into a large ziploc bag.
In a small bowl, whisk the balsamic dressing, olive oil, and minced garlic.
Pour over the peppers and onions and seal the bag.
Turn the ziploc bag, making sure that all of the vegetables are covered with the marinade.
Refrigerate for 1 to 2 hours, turning frequently, for even marinating.
Line a large sheet pan with aluminum foil.
Spray a generous amount of non-stick cooking spray over the foil.
Pour the pepper and onion mixture (including the marinade) over the surface of the pan.
Spread the vegetables out evenly so that they are in one layer.
Roast on the middle rack of the oven for 25 minutes.
Remove the pan from the oven and stir the vegetables.
Sprinkle the crumbled feta cheese over the mixture and return to the oven.
Bake for another 20 minutes
Remove from the oven and serve.