Bring a large pot of salted water to a boil.
Pour 1/4 cup water into a large skillet over medium heat, and add leeks and cauliflower. Cover and cook for 4 minutes, until cauliflower is just becoming tender. Start cooking pasta. Drizzle olive oil into the pan with the cauliflower, and toss with salt and pepper. Continue cooking until cauliflower and leek start to color a bit and cauliflower becomes fork tender, about 5 minutes.
Drain the pasta, reserving 1/4 cup water. Pour the pasta water into the pan with the cauliflower, and bring to a boil. Remove from heat, and swirl in the butter, lemon zest and juice. Pour pasta back into the pot, and scrape cauliflower mixture over it. Mix very well using tongs and a stiff spatula. Add cheese, and toss again.
Divide the pasta mixture among four plates, and top with flake sea salt, chives and additional cheese if desired.