Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the broccoli rabe and cook until just tender, about 5 minutes. Remove with a slotted spoon and set aside. Return the water to a boil and stir in the macaroni. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through but is still firm to the bite, about 8 minutes. Drain well in a colander set in the sink.
While the pasta is cooking, cook and stir the walnut halves in a frypan over medium heat until fragrant, about 5 minutes. Set the walnuts aside, add the olive oil and reduce heat to low. Stir in the garlic and cook until golden brown, about 3 minutes. Stir in the broccoli rabe and cook 3 minutes to reheat. Season to taste with salt and black pepper, then stir in the drained pasta and walnuts. Toss with Parmesan cheese before serving.