Preheat oven to 350 degrees and spray a 8 x 8" baking dish with nonstick cooking spray.
Peel and thinly slice potatoes using a mandoline or with a sharp knife
4 russet potatoes
Slice onion on mandoline or with a sharp knife
½ onion
Place a layer of potatoes in casserole dish
Add a layer of onions
Sprinkle with 2 tablespoons of flour and ¼ teaspoon of salt and pepper
Top with 1 cup of ham and ½ cup of cheese
Repeat layers
Add a third layer of potatoes
Mix dijon mustard with a small amount of milk in a large glass measuring cup. Slowly add remaining milk, stirring as you add. Sprinkle in dried thyme. Pour milk mixture over prepared casserole.