Make the bechamel sauce by heating the oil. Add the leeks and garlic, season with salt and pepper, and saute, stirring, until the leeks are soft, about five minutes.
Add the sausage and herbs and continue to stir-fry about two or three minutes. Add the flour, mix well, and stir-fry another two minutes.
Add half the cashew milk and stir into the sausage mixture until it begins to thicken. Add the remaining cashew milk and mix well. Bring the sauce to a boil over medium heat and turn off.
Preheat the oven to 350 degrees Fahrenheit.
Spread a couple of ladles of the bechamel sauce on the bottom of a 12 X 8 inch baking pan, then layer five noodles over the sauce to cover the bottom of the pan.
Spread half of the chopped spinach over the noodles, then spread a third of the remaining bechamel on top of the spinach. Sprinkle a third of the cheese and a third of the scallions on top, then layer on another five noodles.
Repeat with the other half of the spinach, a third of the bechamel, a third of cheese and a third of scallions and layer on the last five noodles.
Ladle on the remaining bechamel on top of the noodles, sprinkle on the remaining scallions and cheese, and cover with aluminum foil. If you plan on baking this the next day, you can cover tightly with cling wrap at this time and refrigerate overnight. The next day, before baking, remove the cling wrap and replace with foil.
Bake in the preheated oven for 45 minutes, then remove the foil cover and continue baking another 15 minutes until golden on top.
Let stand for five minutes, then cut and serve.