1. Preheat and Prepare:

• Preheat your oven to 350°F (175°C).
• Grease a 12-inch oval or 9×13-inch rectangular casserole dish to prevent sticking.

2. Prepare the Potatoes:

• Wash and scrub the potatoes thoroughly.
• Slice them thinly, about 1/8 inch thick, using a mandoline or sharp knife.

3. Make the Cheese Sauce:

1. In a medium saucepan, melt the butter over medium heat.
2. Add diced onions and minced garlic, stirring frequently for 1–2 minutes until softened.
3. Sprinkle in the flour and whisk continuously to form a roux. Cook for 1 minute to eliminate the raw flour taste.
4. Slowly whisk in the half-and-half, milk, chicken broth, thyme, and salt. Stir until smooth and bring to a simmer.
5. Cook the mixture until it thickens slightly, about 2 minutes. Remove from heat and stir in 1 cup of cheddar cheese and 1 cup of mozzarella cheese until melted. Discard the thyme stems.

4. Layer the Casserole:

1. Spread a thin layer of cheese sauce on the bottom of the prepared casserole dish.
2. Add 1/3 of the potato slices in an even layer.
3. Spoon 1/3 of the cheese sauce over the potatoes and layer with 1/2 of the ham.
4. Repeat the process for a second layer (potatoes, cheese sauce, ham).
5. For the final layer, arrange the remaining potatoes and top with the rest of the cheese sauce. Sprinkle the remaining 1/2 cup of mozzarella cheese on top.

5. Bake:

1. Place the casserole dish on the middle oven rack. Position a cookie sheet on a lower rack to catch any drips.
2. Bake uncovered for 1 hour and 30 minutes, or until the potatoes are tender when pierced with a knife.
3. If the top begins to brown too much, cover loosely with aluminum foil.

6. Rest and Serve:

• Remove the casserole from the oven and let it rest for 15 minutes to allow the layers to set.
• Garnish with parsley or chives, slice, and serve.