Bring a large pot of salted water to a boil and cook your pasta until just past al dente, about 8-10 minutes. Before draining, reserve a cup of starchy pasta water, then drain and set aside.
Meanwhile, cut the cleaned leeks into thin half-moons, making sure to rinse away any grit. Set aside.
In a large skillet, melt the butter over medium-low heat until it begins to foam softly and smells nutty. Add the sliced leeks and a pinch of salt, stirring gently to coat them in the butter.
Cook the leeks slowly over low heat, stirring every few minutes, until they soften and turn a beautiful golden color, about 15-20 minutes. You’ll notice a sweet aroma and the leeks will melt into silky strands.
Pour the heavy cream into the skillet with the caramelized leeks, stirring gently to combine. Let the mixture warm through and thicken slightly, about 2-3 minutes.
Stir in the grated Parmesan cheese and lemon zest, tasting and adjusting with salt and pepper. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it and make it silky.
Gently toss the cooked pasta into the skillet with the sauce, mixing well to coat every strand or piece. Continue to stir until the sauce is glossy and clings beautifully to the pasta, about 1-2 minutes.
Check the seasoning, adding more salt or pepper if needed. Serve immediately, garnished with extra Parmesan or fresh herbs if desired. The pasta should be silky, golden, and fragrant with citrus and caramelized leeks.
