Step 1: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish with butter or non-stick spray.
Step 2: In a medium saucepan, melt the 1/4 cup of butter over medium heat. If using onions and garlic, sauté them in the butter until translucent (about 5 minutes).
Step 3: Whisk in the flour and cook for 1-2 minutes to create a roux, ensuring it doesn’t brown.
Step 4: Gradually whisk in the milk, adding a small amount at a time and whisking until smooth to prevent lumps. Continue cooking until the sauce thickens and bubbles.
Step 5: Stir in the salt, black pepper, and nutmeg. Remove from heat.
Step 6: Arrange one-third of the sliced potatoes in the bottom of the prepared baking dish. Pour one-third of the cream sauce over the potatoes.
Step 7: Repeat the layering two more times until all potatoes and sauce are used.
Step 8: Cover the dish with aluminum foil and bake for 30 minutes.
Step 9: Remove the foil and continue baking for another 30-40 minutes, or until the potatoes are tender when pierced with a fork and the top is golden brown.
Step 10: Let the dish rest for 10-15 minutes before serving to allow the sauce to set.