Preheat your oven to 375°F. Grease the baking dish lightly with butter so nothing sticks during baking time.

Wash and peel potatoes. Slice them evenly and thin. Thin slices cook faster and become soft and creamy without hardness inside.

Melt butter in a saucepan. Add onion and garlic. Cook slowly until soft and fragrant, but do not brown them.

Sprinkle flour over cooked onions. Whisk well. Slowly add milk. Keep whisking so no lumps form inside the creamy sauce.

Add salt, pepper and thyme. Let the sauce simmer gently. It should become slightly thick and smooth, not watery anymore.

Place half the potato slices in the baking dish. Spread them evenly so every bite has tender potato layers.

Spread diced ham over potatoes. Try to cover most areas. This gives salty smoky flavor in every delicious forkful.

Sprinkle cheese on top. The cheese will melt and add rich creamy taste. It helps bind potatoes and ham together.

Pour half of the sauce over layers. Make sure the sauce seeps down. It helps everything cook evenly and stay moist.

Add remaining potatoes, ham, cheese and sauce. Keep layers even. More cheese on top gives golden crust after baking.

Cover dish with foil. Bake for about 45 minutes. This helps potatoes soften without burning top too early.

Remove foil and bake 20–25 more minutes. The top should turn golden and bubbly. Check potatoes with fork for tenderness.

Let casserole rest 10 minutes. This helps it set and slice cleanly. Hot sauce thickens slightly while resting time passes.