Bring a large pot of water to boil and salt it. Prepare an ice bath (a few handfuls of ice and some cold water in a bowl). 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. Put the broccoli rabe in the ice bath to stop the cooking, and drain again. At this point, you can refrigerate the broccoli rabe, wrapped well or in a covered container, for up to 2 days.
Cook the sausages over medium heat in a large skillet, pricking them with a fork a few times and turning from time to time, until browned in places, 10 to 15 minutes.
Remove the sausages from the skillet (don’t worry if it’s not done) and cut them into bite-sized pieces. Return to the skillet over medium heat; cook, turning occasionally until all sides of the sausage are nicely browned, about 5 minutes more.
Squeeze the excess liquid from the broccoli rabe; chop it coarsely. Add it to the skillet along with the garlic and cook, stirring occasionally, until it’s fully tender and browned in places, 3 to 4 minutes. Add the grapes, sprinkle with salt pepper, and cook until the grapes are heated through. Taste and adjust the seasoning. Serve immediately.