Preheat oven to 275°F. Line a rimmed baking sheet with heavy foil, matte side up.
Combine balsamic, sugar, chilies, garlic, herbs and grated onion in a large, shallow dish. Season ribs with salt and pepper liberally, toss with dressing and arrange on the baking sheet. Top with excess dressing and cover with more foil to seal as a packet. Roast 3 hours.