Preheat your oven to 375°F. Butter a 9x13-inch baking dish and set aside. Peel the 3 lb russet potatoes peeled and sliced 1/8-inch thick and slice them 1/8-inch thick — a mandoline is ideal for even, uniform slices, but a sharp knife works too. Keep the slices in a mixing bowl of cold water while you prep to prevent browning, then drain and pat dry thoroughly with a clean towel before layering.

In a large saucepan over medium heat, melt the 4 tbsp of unsalted butter. Add the diced yellow onion thinly sliced into half-moons and cook, stirring occasionally, until soft and translucent, about 5 minutes. Add the minced cloves garlic minced and cook for 1 more minute until fragrant.

Sprinkle the ¼ cup all-purpose flour over the 4 tbsp unsalted butter and 1 yellow onion thinly sliced into half-moons mixture. Stir constantly with a whisk or wooden spoon for 1–2 minutes to cook out the raw flour taste — the mixture should look like a pale paste.

Slowly pour in the warmed 2 cups whole milk and 1 cup heavy cream, whisking constantly to prevent lumps. Add the 1 ½ tsp kosher salt, ½ tsp black pepper freshly ground, ¼ tsp ground nutmeg, and 1 tsp fresh thyme leaves (if using). Continue whisking over medium heat until the sauce thickens enough to coat the back of a spoon, about 5–7 minutes. Taste and adjust seasoning. Remove from heat.

Arrange one-third of the russet potatoes peeled and sliced 1/8-inch thick slices in an even, slightly overlapping layer in the prepared baking dish. Scatter one-third of the sliced yellow onion thinly sliced into half-moons over the top. Pour one-third of the cream sauce evenly over the layer. Repeat this process two more times — potatoes, onion, sauce — finishing with the sauce on top. Dot the surface with the small pieces of unsalted butter.

Cover the dish tightly with aluminum foil. Bake at 375°F for 45 minutes. The foil traps steam and ensures the 3 lb russet potatoes peeled and sliced 1/8-inch thick cook through evenly before the top browns.

Remove the aluminum foil and continue baking for an additional 25–30 minutes, until the top is deeply golden and the 3 lb russet potatoes peeled and sliced 1/8-inch thick are completely tender when pierced with a knife. The sauce should be bubbling around the edges.

Remove from the oven and let the dish rest for 10–15 minutes before serving. This allows the sauce to thicken and set, making it much easier to portion. Garnish with fresh chopped fresh parsley chopped if desired.