Preheat oven to 350°F. Butter a 9x13 inch baking dish.

Peel potatoes and slice to 1/8-inch thickness using a mandoline or sharp knife. Place slices in cold water as you work, then drain well and pat dry with paper towels.

In a large saucepan, melt butter over medium heat. Add diced onion and cook until soft and translucent, about 5 minutes. Add minced garlic and cook for 30 seconds more until fragrant.

Sprinkle flour over the onion mixture and stir constantly for 1-2 minutes to cook the flour.

Gradually whisk in the milk and cream, a little at a time, ensuring there are no lumps. Bring to a gentle simmer and cook, stirring constantly, until thickened enough to coat the back of a spoon, about 3-5 minutes.

Remove from heat and stir in 1 1/3 cups of the cheese mixture until melted. Add salt, pepper, thyme, and nutmeg. Taste and adjust seasonings if needed.

Layer one-third of the potatoes in the prepared baking dish, slightly overlapping. Pour one-third of the cheese sauce over the potatoes. Repeat two more times with remaining potatoes and sauce.

Sprinkle the remaining 2/3 cup cheese over the top. Cover tightly with aluminum foil.

Bake for 45 minutes, then remove foil and bake for an additional 30-40 minutes until potatoes are tender and top is golden brown and bubbling.

Allow to rest for 15 minutes before serving. Garnish with fresh parsley if desired.