Preheat the oven to 350 degrees F.
With the skin on, cut the potatoes into ¼ inch thick slices. It’s best to cut the potatoes just before putting them in the baking dish because they’ll turn a light orange color fairly quickly after cutting them. If cutting the potatoes ahead of time put them in a bowl of water until just before use.
In a baking dish, drizzle olive oil and spread it around with a paper towel or baking brush.
Add a layer of sliced potatoes to the baking dish. Then add a layer of onions, salt, pepper, sage garlic, ham, and both types of shredded cheese.
Then repeat by adding another layer of the same ingredients.
Pour half-and-half evenly over the top. Then spread the cubes of butter evenly on the top.
Cover with foil and bake in a preheated oven for 40 minutes.
Then remove the foil and bake for another 20 minutes or until the potatoes are bubbling hot.
Serve warm and garnish with parsley.