Thinly slice half the lemon into rounds, and set the other half aside. Toss the lemon slices, garlic, and broccolini with the olive oil on a rimmed baking sheet. Season with salt and pepper, and make sure everything is evenly coated, especially the broccolini tips so they all get fried and crispy.
Sprinkle with the Parmesan and roast until the broccolini is bright green and starting to char and the cheese is golden brown, 10 to 15 minutes. Remove from the oven, squeeze the other half of the lemon over the top, and serve.
