Preheat Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray. Prepare Potatoes and Ham: Peel and thinly slice the potatoes. Dice the ham into bite-sized pieces. Make the Cheese Sauce: In a large saucepan or skillet, melt the butter over medium heat. Add the minced garlic and sliced onions, and sauté until onions are softened and translucent.

Add Flour and Create Roux: Sprinkle the flour over the butter and onion mixture, stirring constantly to form a paste (roux). Cook for 1-2 minutes until the flour is lightly golden.

Add Milk and Cream: Gradually pour in the milk and heavy cream, stirring constantly to avoid lumps.

Cook and stir until the mixture begins to thicken, about 5-7 minutes. Season with salt and black pepper to taste.

Layer Potatoes, Ham, and Cheese Sauce:

Place half of the sliced potatoes in the prepared baking dish. Sprinkle half of the diced ham over the potatoes.

Pour half of the cheese sauce evenly over the potatoes and ham. Repeat with the remaining potatoes, ham, and cheese sauce.

Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 60-75 minutes, or until the potatoes are tender when pierced with a fork.

Add Cheese and Finish Baking:

Remove the foil and sprinkle shredded cheddar cheese evenly over the top.

Return the dish to the oven and bake uncovered for an additional 10-15 minutes, or until the cheese is melted and bubbly.

Serve: Remove from the oven and let it rest for 10 minutes before serving. Garnish with fresh thyme leaves if desired.