RECIPE PREP: Preheat oven to 350ºF. Parboil the sliced potatoes. Drop them into a pot of heavily-salted boiling water. Cook for 6 minutes. Drain off the water and pat the potatoes dry. Lightly grease a 9×9-inch pan with butter.

1. Add butter to a saucepan over medium heat. Once it’s melted, add garlic and rosemary to the pan. Stir and cook 1 minute. Next, sprinkle in flour and whisk until a paste forms. Cook 1 minute. Slowly add in cream, whisking constantly. Continue whisking until no large lumps of roux remain.
2. Reduce heat to medium-low. Cook the mixture, stirring often, until it’s bubbling and slightly thickened. Keep an eye on the sauce as heavy cream can easily bubble over. Stir in the sour cream, 1/2 cup colby jack, 1/2 cup cheddar, salt and pepper. Stir until the cheese has fully melted into the sauce.
3. Spoon a bit of cheese sauce over the bottom of the greased baking dish. Arrange the potato slices standing upright in 3 rows inside the dish. (The vertical position of the slices will allow the cheese sauce to penetrate the layers.) Season with salt and pepper along the way. 
4. Sprinkle diced ham or bacon over the potatoes (optional). Pour cheese sauce over the meat and potatoes so everything is covered. Depending on the height of your baker, you may have some sauce left over. Cover the dish with aluminum foil and bake for 25 minutes.
5. Uncover the dish. Top with parmesan cheese, and remaining cheddar cheese and colby jack. Bake uncovered for 20 to 25 minutes, or until the top is golden and the potatoes are fork tender. Optional: If the top needs crisping, change the oven setting to broil for 5 minutes before removing the dish from the oven. 👉🏻 But keep a close eye on things to prevent scorching.
6. Allow the dish to cool for 15 minutes before serving. It’ll give the cheese sauce time to thicken. Garnish with remaining ham or bacon, and fresh herbs or green onion.