- 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.