Bring a large pot of salted water to a rolling boil. Add your pasta and cook until just shy of al dente, about 1 minute less than package directions. Reserve a cup of starchy pasta water, then drain and set aside.
While the pasta cooks, rinse and thinly slice the leeks. Make sure to wash thoroughly to remove any grit trapped between layers.
Heat a wide skillet over medium-low heat and add the olive oil. When shimmering, add the sliced leeks and cook slowly, stirring occasionally, until they soften and turn golden, about 10 minutes. The aroma should be sweet and caramel-like.
Add the minced garlic to the leeks and cook for about 30 seconds until fragrant, being careful not to let it brown.
Toss in the chopped spinach in batches, stirring until each addition wilts down and becomes glossy, about 2-3 minutes. The greens should be vibrant and tender.
Reduce the heat to low, then add the drained pasta to the skillet. Pour in a splash of the reserved pasta water and toss gently to coat the noodles in the flavorful greens and garlic oil. If needed, add more pasta water to loosen the mixture into a silky sauce.
Season generously with salt and freshly cracked pepper, tasting and adjusting as needed. Finish by adding the lemon zest over the pasta and tossing gently to distribute evenly.
Serve immediately, drizzled with a little extra olive oil if desired, and enjoy the vibrant, glossy greens combined with tender pasta. The dish is best enjoyed hot, with the flavors freshly melded.
