1 yellow onion, sliced into slivers, lengthwise (with the grain)
1 large bunch of broccoli raab, rinsed and cut into 2-inch long pieces
2-3 garlic cloves, sliced
1/4 teaspoon red chili flakes
Freshly ground black pepper
Heat 2 Tbsp olive oil in a large sauté pan on medium heat. Add the onions, spread out in a thin layer. Cook, stirring occasionally until softened and then lightly browned. (Tip: to speed up the caramelization process you can sprinkle a pinch of sugar over the onions.) If the onions start to dry out at all, lower the heat (you can add a little water to them too.) They should brown, but not get dried out.
After you start the onions, bring a large pot of water to a boil. The onions take at least 15 minute to cook, so you'll have time to get the water boiling. Salt the water (about a tablespoon of salt for 3 quarts of water). Prepare an ice bath, fill a large bowl half way with ice water. Add the raab to the boiling water. Blanch for 1 minute. Use a slotted spoon to remove from the boiling water and put in the ice bath to stop the cooking. Shocking the raab with ice water will also help keep the raab bright green colored.
Note that some people blanch their raab, some do not. Raab can be rather bitter, so blanching will help take the edge off of the bitterness. If your raab isn't particularly bitter, or you like bitter greens, you can easily skip this blanching step.
Drain the ice water from the raab. Use a clean tea towel to gently wring out the excess moisture from the raab.
Once the onions are lightly browned, remove them from the pan to a bowl. Using the same pan, add another Tbsp of olive oil and heat the pan on high heat. Add the chili flakes. Once the chili flakes start to sizzle, add the garlic. Once the garlic just starts to brown at the edges add the broccoli raab and the onions. Toss the raab mixture so that it gets well coated with the olive oil. Cook on high heat until most of the moisture is gone, about 5 minutes if you blanched first, a minute or too longer if you skipped the blanching.