COOK onions in large nonstick skillet on medium heat 10 min. or until caramelized. Remove from skillet; cover to keep warm.
ADD meat to skillet in batches; cook 2 to 3 min. or until done, turning occasionally and adding chiles to skillet with the last batch of meat. Combine all meat, onions and cilantro in bowl; cover to keep warm.
MIX mayo and garlic powder; spread onto cut sides of bolillos. Cook in skillet 2 min. on each side or until golden brown on both sides. Fill with meat mixture and Singles.