Broccoli Rabe With Caramelized Onions

Name: Broccoli Rabe With Caramelized Onions
Ingredients: Olive oil; 1 yellow onion, sliced into slivers, lengthwise (with the grain); 1 large bunch of broccoli rabe (raab, rapini), rinsed and cut into 2-inch long pieces; 2-3 garlic cloves, sliced; 1/4 teaspoon red chili flakes; Salt; Freshly ground black pepper