Cut cooked turkey into bite sized cubes. Set aside
Heat a medium fry pan over medium high heat. Add butter. Once butter has just melted and starting to sizzle, add the chopped onion. Saute until soft and just starting to turn translucent. About 5 minutes.
Spray a 9 x 13 Pyrex Baking dish with Pam.
Spread the uncooked elbow macaroni over the bottom of the baking dish.
Spread the cubed turkey over the elbow macaroni.
In a medium sized bowl, whisk together the milk and the cream of chicken soup. Add the cooked onion and stir.
Pour the soup/milk/onion mixture over the turkey and macaroni.
Cover and let sit in refrigerator overnight.
The next day, preheat oven to 350 degrees. Spread the grated cheese over the top of the casserole and bake uncovered for up to an hour. Or until bubbly and hot.
In the meantime, make the panko bread crumbs.
Heat a medium fry pan over medium high heat. Add the butter. Once the butter has almost melted and starting to sizzle, add the panko bread crumbs and seasoning. Turn the heat down to medium. Toast the panko until golden brown. This will require constant supervision and lots of stirring.
Once the casserole is cooked, sprinkle the toasted panko bread crumbs evenly on top. Serve. A tossed salad is a perfect side dish for this casserole.