Bring a pot of water to a boil. Have a bowl of ice water ready. Rinse the broccoli rabe and trim about 1/2 inch off the bottom of the stalks. Blanch the broccoli rabe in the boiling water until bright green and softened, 2 to 3 minutes. Drain.
Place the broccoli in the ice water until completely cooled. Drain again and dry on paper towels.
Heat the oil in a skillet over medium-high heat. Add the broccoli rabe, red pepper, and salt and pepper to taste. Sauté until very tender, 5 to 8 minutes. Add the juice of about half the lemon to the pan in the last minute or so of cooking. Transfer to a plate, squeeze on the remaining lemon and serve.