Start by simply pre-heating the oven to 350 degrees F. While it’s warming up, prepare the rest of the ingredients.
Next, grab a large skillet to which you will add the onions and some butter (not the two tablespoons, just some extra for the pan).
Sauté the onions for a few minutes until they start to go translucent.
Take the opportunity to butter a large casserole dish, as you’ll need it for later.
Once the onions are adequately cooked, add the ham and continue cooking them both together over medium heat. Do this for 3-4 minutes until everything’s thoroughly heated.
Set the pan aside for later.
Next, get a microwave-safe container, and add the 1 ½ cups of half and half and warm it up for about a minute until it's a little warmer than room temperature.
Using the same container, add in the flour and black pepper, and give it a good stir so they are thoroughly combined, and then set aside.
Pro tip: Depending on how salty your ham is, you can also add some salt at this stage, too. Generally speaking, ham is already quite salty, so check the flavor first and use salt sparingly.
Using a small Tupperware or bowl, add in the grated cheddar cheese and grated Monterey Jack Cheese, and give them a good mix.
Now you need to cut the potatoes. A mandolin works best for this, but you can also just use a knife and cut them carefully. We are looking for extremely thin slices here at about 1/8th of an inch.
To arrange/combine everything together, take the casserole dish you buttered earlier and create a 3-stagelayer using 1/3rd of the baked potato slices to line the bottom, 1/3rd of the onion and ham mixture, 1/3rd of the cheese mixture, and 1/3rd of the cream/seasoning mixture.
Repeat this two more times to create a 9-layer casserole.
Now it’s time to bake! First, cover the assembled casserole with foil and bake for 40 minutes.
Then, remove the foil and bake for an additional 20 minutes or until the top is golden and bubbly.
Cut into squares, top with fresh parsley, and it’s ready to serve!