Prepare the broccoli rabe by trimming and discarding the tough lower stems, then coarsely chopping the leaves and tender stems.
Bring a large pot of salted water to a boil and cook the broccoli rabe for 5-7 minutes until tender. Immediately transfer it to a bowl of ice water to stop the cooking process, then drain thoroughly.
Meanwhile, cook the penne in a large pot of boiling, lightly salted water according to package directions until al dente.
Before draining, carefully ladle out and reserve about 3/4 cup / 180 ml of the starchy pasta water.
Drain the pasta and return it to the warm pot.
In a large skillet, heat the olive oil over medium heat.
Add the minced garlic and crushed red pepper, stirring constantly for about 1 minute until fragrant—be careful not to let the garlic burn.
Add the drained broccoli rabe to the skillet and toss to coat it in the garlic-infused oil.
Stir in the cooked pasta, the reserved 3/4 cup / 180 ml pasta water, the 1/4 cup / 25 g (0.88 oz) grated Parmesan, and the lemon juice.
Continue to cook and stir everything together for 2-3 minutes, until the sauce has slightly thickened and everything is heated through.
Season generously with kosher salt and freshly ground black pepper to your liking.
Serve immediately, topped with the remaining 1/3 cup / 30 g (1.06 oz) shredded Parmesan cheese.
Don't skip blanching the broccoli rabe in boiling water and shocking it in an ice bath. This crucial step helps to mellow its natural bitterness and locks in its vibrant green color.