The below will make a 9 inch casserole.
Sauce: This is a type of Morney sauce (or Béchamel sauce with cheese). I try to make Béchamel with the least amount of fat possible. To do this I start with finely chopped onions before adding the flour. Because the flour coats the surface of the chopped onion, it will make a smooth Béchamel without much fat.
I sautéed 1 medium finely chopped onion in a frying pan (2 tbs of light olive oil instead of butter) and seasoned it with salt. I then added shiitake mushroom (optional 3 big ones, stem removed and finely chopped). After mushrooms were softened, I added flour (3 tbs) stirring until dry flour was no longer visible and the pieces of vegetable were coated with flour. I added cold 1% (instead of cream or 4%) milk at once (about 1 cup and add more later). I stirred the mixture with a silicon spatula until the sauce thickened. Since cheeses will be added and the noodles may absorb moisture, I wanted this sauce to be rather runny. I added more milk until the desired consistency was attained. I seasoned it with salt, white pepper and freshly grated nutmeg (we like lots of freshly grated nutmeg).
Cheese: We added two cheeses; sharp Cheddar (1/2 cup grated) and Gruyere cheese (1/2 cup grated). If the sauce became too thick, you can add more milk to loosen it.
Chicken: This is optional but one of the reason for this dish was to use the leftover barbecued chicken. I used about 1/2 cup of cooked and shredded chicken which was mixed into the sauce. Since the chicken was hot smoked, it added a nice smoky flavor.
Noodles: This was Pennsylvania Dutch noodle (wide noodle) cooked. I added about a cup to the chicken cheese mixture. I placed the sauce mixture with noodles into the casserole and placed it in a 400F preheated convection (toaster) oven for 15-20 minutes or until the surface started showing brown spots. I grated Riggiano Palmigiano cheese and chopped parsley on the top. I let it rest for 10 minutes before serving.
This was a quite wonderful “comfort” dish for leftover control. Often Cheddar cheese cooked too long becomes “chalky” but the addition of Gruyere and the rather loose sauce appeared to prevent this from happening. The nutmeg and smoky chicken added nice flavor. I could have added bread crumbs on the top to make crunchy surface but even without it this was a fine dish.