Bring a large pot of salted water to a boil. Add the pasta, adjust the heat to maintain a gentle boil, and cook until just shy of al dente, 2 to 3 minutes less than the package instructions. Reserve 1 cup of pasta water. Drain the pasta, then return to the pot on the stove over medium-low heat.
Stir in ¾ cup of the pasta water and half the lemon juice. Add several grinds of black pepper, then gradually add the Parmesan, stirring vigorously between additions to prevent clumping, until a silky sauce has formed. Add a tablespoon of water at a time as needed to make a creamier sauce if it looks dry. Stir in the labneh, remaining lemon juice, lemon zest and scallions until creamy and combined, about 1 minute more. Season with salt as needed. Finish with another grind of pepper and serve immediately.