Boil the lasagna noodles according to package directions. When done, drain and set aside.
In a medium sauce pan over medium heat melt the butter for the sauce. Whisk in the flour and as soon as it browns (about 1 to 2 minutes) whisk in the milk. Bring the milk to a very light boil and whisk occasionally until it thickens, about 10 to 15 minutes.
While your noodles and sauce are cooking go ahead and work on the filling. In a large bowl mix together the chicken, spinach, cream cheese, breadcrumbs, salt, pepper, nutmeg, garlic, parsley and 1/2 cup of the parmesan cheese. Use your hands if you need to in order to make sure it's mixed thoroughly.
In a rectangular casserole dish layer the ingredients in this order sauce, noodles, sauce, chicken mixture, noodles, sauce, chicken mixture, noodles, and then sauce. Any noodles on the top that don't get covered in sauce could get crispy so be careful about even coverage. Top the whole thing off with the other 1/2 cup of parmesan cheese. Since I sometimes get confused about the order to layer lasagna ingredients I made you a little chart for this recipe (see the notes section).
Bake at 400 degrees until brown on top, about 15 to 20 minutes. If your casserole was cold when you started (either from the fridge or defrosted completely from the freezer) then you'll need to increase the baking time.