Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.

In a medium saucepan, melt butter over medium heat. Add the minced garlic and cook for 1 minute. Stir in the flour and cook for another 1-2 minutes to form a roux.

Gradually whisk in the cream or milk, and continue whisking until the sauce thickens (about 3-4 minutes). Add salt, pepper, and nutmeg (if using). Remove from heat and stir in 1 cup of shredded cheddar cheese until melted.

Layer half of the thinly sliced potatoes in the prepared baking dish. Pour half of the cheese sauce over the potatoes. Repeat with the remaining potatoes and sauce.

Cover the dish with aluminum foil and bake for 45 minutes. Remove the foil, sprinkle the remaining 1/2 cup of cheddar cheese on top, and bake uncovered for another 20-25 minutes, until the potatoes are tender and the cheese is golden and bubbly.

Let the scalloped potatoes cool for 5-10 minutes before serving. Garnish with fresh parsley if desired.