Bring a large pot of water to a boil and salt it. Prepare an ice bath for shocking. Add the broccoli rabe to the boiling water and cook, stirring once or twice, until it is bright green and beginning to get tender, about 3 minutes. Drain. Shock in the ice bath and drain again. At this point, you can refrigerate the broccoli rabe, wrapped well or in a covered container, for up to 2 days.
Put 3 tablespoons of olive oil in a large skillet over medium heat. When it is hot, add the garlic and anchovies, and a pinch of red chile flakes if you like, and cook for about 2 minutes before adding the broccoli rabe.
Squeeze the excess liquid from the broccoli rabe. chop it coarsely. Add it to the skillet and cook, stirring occasionally, until it’s fully tender and browned in places, 3 to 4 minutes. Taste and adjust the seasonings and serve immediately.