Slice the leeks down the middle lengthwise. Thinly slice into half-moons. Wash well - leeks tend to hold a lot of sand and dirt. Try to get them as dry as possible.
Add the leeks, pasta, garlic, red pepper flakes, oil, 2 teaspoon salt, ½ teaspoon pepper, and water in a large skillet. The pasta should lay flat. (Use a deeper skillet than the pan shown in the photo.)
Bring to boil over high, simmer, stirring and flipping the pasta frequently. Cook until the water has almost completely evaporated, ~9-11 minutes. Turn off.
Add lemon zest and parsley. Toss to coat.
Taste. Add salt, pepper, and red pepper flakes as desired.