Bring a large pot of water to a rolling boil.
Pre-heat the oven to 375°F.
Arrange the bacon on a slotted broiler pan and bake until crisp, 15-18 minutes. Remove and chop; reserve.
Heat the oil in a skillet over medium heat; add the leeks and garlic and sauté for 5 minutes. Deglaze with the wine or stock; allow the liquid to almost evaporate and reserve over low heat.
In a saucepot over medium heat, melt the butter and whisk in the flour. Cook for 1 minute, then add the warm milk and simmer at a gentle bubble to thicken. Season with salt, pepper and nutmeg.
Season the boiling water with salt and cook the pasta to al dente.
Stir the Swiss or Gruyère and cheddar into white sauce; remove from the heat and stir in the Dijon.
Combine the pasta with the bacon, leeks and cheese sauce. Place in a baking dish or casserole and top with the Parmigiano Reggiano. Brown under the broiler. For a make-ahead meal, cool and bake off from room temperature at 375°F until brown and bubbly.