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 4 tbsp unsalted butter. Add the diced 1 yellow onion thinly sliced into half-moons and cook, stirring occasionally, until soft and translucent, about 5 minutes. Add the minced 3 cloves garlic minced and cook for 1 more minute until fragrant.

Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.

Slowly pour in the warmed 2 cups whole milk warmed and 1 cup heavy cream warmed, 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 or ½ tsp dried thyme (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.

Peel and slice onions to about 1/8-inch thick — thin enough to cook quickly, thick enough to avoid scorching. Yellow or Spanish onions work best. Limit to 2 large onions per 12-inch Skillet to prevent steaming. Have your fat ready and keep a splash of water, broth, or wine nearby to deglaze if needed.

Heat a Skillet over medium to medium-high heat. Add butter, oil, or a mix and let it shimmer. Add sliced onions in large handfuls, stirring for 1–2 minutes per batch until they begin to soften — this prevents overcrowding. Once all onions are in, season with a pinch of salt. Cook over medium heat, stirring every few minutes, for 8–12 minutes total until soft, translucent, and lightly golden. To speed up softening, add 1/4 to ½ cup water or broth and cover; steam wilts the onions in a few minutes. Remove the lid once liquid evaporates and continue cooking uncovered. If the pan looks dry, deglaze with a splash of water, broth, or wine, scraping up any browned bits. Don't crank the heat — high heat burns the outside before the onions cook through.

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.