1. Heat oil in a large frying pan over medium-high heat. Cook garlic until lightly browned, 3 minutes. Add onion and 1/2 tsp. each salt and chile flakes and cook until onions are translucent, about 5 minutes more. Add vinegar and broccoli rabe, stirring to coat.
2. Reduce heat to medium-low and add 1/2 cup water. Cook, covered, until thick stems are fork tender, 15 to 20 minutes. Stir in remaining 1/2 tsp. each salt and chile flakes.