In 12" nonstick fry pan over medium heat, melt 4 Tbs. butter. Add leeks and salt, stirring to coat leeks with butter. Cover and cook, stirring occasionally, until leeks are tender, about 20 minutes.
Add thyme, nutmeg, pepper, and cream. Simmer, uncovered, stirring occasionally, until thickened, about 15 minutes. Transfer to bowl. Let cool. Wipe out pan, grease, with 1 Tbs. butter.
Preheat oven to 400 degrees F.
In bowl, combine cheeses. Layer one-third of potatoes in fry pan. Spread one-third of leek mixture on top. Sprinkle with one-third of cheeses, then
1 Tbs. chives. Repeat layering, two more times, reserving 1 Tbs. chives. Cover pan; transfer to oven. Bake 45 minutes. Remove lid and bake until potatoes are tender and crust is golden brown, about 30 minutes more.
Sprinkle remaining chives on top. Let stand 15 minutes before serving.