Go ahead and prep by preheating the oven to 400 Degrees Fahrenheit & spray your glass casserole Dish with cooking Spray.
Dice your Onion into finely Chopped pieces and your ham into bite sized bits.
Using either a knife or Mandolin slice your potatoes into thin slices (I Leave the skin on, but you can peal it if you choose).
Mix the cream of Celery, Milk, and Seasoned salt together.
To start the layering process start by spreading a thin layer of the cream of celery milk mixture in the bottom of the dish, then a simple layer of Potatoes. Then add a layer of Onion, then Ham, then Cheese, and finish with a layer of cream of celery milk mixture.
Keep layering in that order till you are out of ingredients or the dish is full.
Then take your butter and chop into small bits and spread about the top of the dish and cover with Aluminum Foil.
Bake for an Hour and 20 minutes, then remove the foil and continue baking for the last 10 minutes, or longer depending on how much you would like it browned.
The next step is the keep yourself from eating the whole dish yourself because it's going to taste amazing. Good luck with your cheesy adventure!