In a large skillet, warm the olive oil. Add the leeks and saute for a few minutes until softened. Stir in the mushrooms and a half teaspoon salt and cook over medium heat until the mushrooms are cooked through, another 10-15 minutes. Taste for seasoning and add more salt if needed and some black pepper.
Meanwhile, pour a cup of boiling hot water over the porcini mushrooms and let them rehydrate for 10-15 minutes. Pull from the liquid, coarsely chop, and pour ½ cup of the soaking liquid in with the milk leaving any sediment on the bottom of the bowl from the mushrooms. Put the porcini in with the leeks and mushrooms, take off the heat and set aside.
In a medium saucepan, melt the butter over low heat and add the garlic. Add the flour and cook, stirring constantly, for 3-4 minutes. Slowly pour in the milk mixture and switch to a whisk. Whisk over medium heat until it comes to a simmer. Simmer, without stirring, over low heat for 2 minutes, then take off the heat and stir in the cheese, nutmeg and 1 teaspoon of salt. Taste and adjust seasoning if needed.
Cook the pasta according to package directions in plenty of salted water, until al dente (since the pasta will cook more in the oven, boil it shy of 2 minutes according to when it will be finished). Drain and dump back into the pasta pot, not over the heat.
Pour the bechamel in with the pasta and stir to coat. Add the mushrooms and combine. Pour into a buttered 9 x 13 inch baking dish. You can refrigerate it at this point up to 8 hours before baking.
In a small bowl, combine the breadcrumbs, parsley and ¼ teaspoon salt. Scatter over the pasta evenly. Place dish in a preheated 325 degree oven. Bake for 40-45 minutes, until top is toasted and it is warm throughout. Serve immediately.