Prepare the Béchamel sauce:
Melt butter in a saucepan and stir in sifted flour. Cook for 1 minute, whisking constantly. Add milk in a stream and bring to a boil, whisking constantly. Season with salt and nutmeg to taste. Cook for a few minutes until thick.
Clean leeks and discard green part. Cut in half lengthwise and each half into 1/8-inch slices. Cut ham into 1/4-inch cubes. Heat oil in a pan over medium heat and sauté leeks for 2 minutes. Add ham and cook for 2 more minutes, stirring.
Pour in wine, allow to evaporate completely and continue cooking for another 5 minutes. Season with salt and pepper.
Boil pasta in plenty of salted water according to package directions.
Drain and toss with the leek sauce, warm Béchamel sauce and Parmigiano. Serve immediately.
