In a large bowl, combine garlic and vinegar; let sit 15 minutes. Meanwhile, halve peppers. Remove seeds and white membranes. Cut peppers into bite-size chunks and set aside. Separate parsley leaves from stems; discard stems (you should have about 1 cup leaves). Finely chop leaves and set aside.
Whisk olive oil, salt, and pepper into garlic-vinegar mixture. Add more salt and pepper to taste.
Toss peppers, parsley, and onion with dressing. Add feta and toss gently. Serve at room temperature, or cover and chill for up to 2 hours.
Note: Nutritional analysis is per serving.