Bring a large pot of water to a rolling boil. Season generously with salt.
Separate the dark green tops of the leeks from the white and light green parts. Place each into its own bowl of cold water and soak for 10 minutes, swishing gently to release trapped dirt. Drain and dry very well.
Heat a large Dutch oven or deep skillet over medium heat. Add oil and the dark green leek pieces with a pinch of salt. Cook, stirring occasionally, until softened considerably, about 5 to 7 minutes.
2 tablespoons oil, 3 large large leeks, 1-2 teaspoons kosher salt
Add the white and light green leek pieces with another pinch of salt. Continue cooking until everything is soft and silky, about 3 to 4 minutes more.
3 large large leeks
Meanwhile, cook the pasta for 2 minutes under the package directions for al dente. Reserve at least 2 cups of pasta water before draining.
16 ounces pasta of choice
Add the butter, garlic, and red pepper flakes to the leeks. Cook, stirring continuously, until fragrant, about 30 seconds to 1 minute.
4 tablespoons unsalted butter, 8 to 10 cloves garlic, ½ teaspoon red pepper flakes
Reduce heat to low. Add the drained pasta and a few ladles of pasta water. Toss to combine.
1 to 2 cups reserved pasta water
Add the parmesan in small handfuls, tossing between each addition and adding more pasta water as needed to keep the sauce glossy and loose.
6 ounces parmesan
Once all the parmesan is incorporated, add the leek butter, lemon zest, and lemon juice. Remove from heat and toss until silky and well combined. Taste and adjust salt and pepper.
2 tablespoons fresh lemon juice, 1 tablespoon lemon zest, 3 tablespoons leek butter, ½ teaspoon coarse ground black pepper