Preheat your oven to 375°F (190°C). Butter or lightly grease a 9x13 inch baking dish and set aside. Slice the peeled potatoes uniformly thin (about 1/8-inch) using a sharp knife or a mandoline. Place the sliced potatoes in a large bowl.

In a large saucepan or Dutch oven, melt the butter over medium heat. Add the finely minced onion and cook for 3-5 minutes until softened and translucent, being careful not to brown. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion and garlic. Cook, stirring constantly with a whisk, for 1 minute to create a roux. Gradually whisk in the warm milk and heavy cream, a little at a time, until smooth and no lumps remain. Bring the mixture to a gentle simmer, stirring constantly, and cook for 2-3 minutes until thickened.

Remove the saucepan from the heat. Stir in the chopped fresh thyme, rosemary, 1/2 tsp salt, 1/4 tsp black pepper, and a pinch of nutmeg (if using). Add 1 cup of the shredded sharp cheddar cheese and 3/4 cup of the shredded Gruyère cheese. Stir until the cheeses are completely melted and the sauce is smooth and creamy. Taste and adjust seasoning if needed.

Pour the prepared cheese sauce over the sliced potatoes in the large bowl. Gently toss the potatoes to ensure every slice is thoroughly coated with the creamy sauce. This helps prevent sticking and ensures even flavor distribution.

Arrange half of the coated potatoes evenly in the prepared baking dish. Top with half of the remaining shredded cheddar and Gruyère cheese (reserve some for the final topping). Layer the remaining potatoes over the cheese, spreading them evenly.

Cover the baking dish tightly with aluminum foil. Bake for 35 minutes. This steaming period allows the potatoes to cook through without drying out.

Remove the foil. Sprinkle the remaining shredded cheese over the top of the casserole. Return to the oven and bake uncovered for an additional 20-25 minutes, or until the potatoes are fork-tender and the cheese is bubbly, golden brown, and slightly crispy.

Once baked, remove the casserole from the oven and let it rest for 10-15 minutes before serving. This allows the sauce to set and makes for easier serving.