✓ Preheat oven to 375°F (190°C).

✓ Peel and thinly slice the potatoes, then soak in cold water for at least 30 minutes.

✓ Rinse the potatoes under cold running water and pat dry thoroughly with a clean kitchen towel.

✓ In a large skillet over medium heat, melt 3 tablespoons of butter and sauté the sliced onions until they are translucent and golden brown (about 15-20 minutes).

✓ Add minced garlic to the onions and cook for another minute until fragrant.

✓ Drain the potatoes and return them to the skillet with the onions and garlic. Stir in heavy cream, milk, nutmeg, Dijon mustard, salt, and pepper. Cook over low heat until the mixture thickens slightly (about 5 minutes).

✓ Grease a baking dish with remaining butter.

✓ Arrange a layer of potato slices in the bottom of the dish, pressing them down lightly to ensure they stick together.

✓ Spread half of the cheese mixture evenly on top of the potatoes.

✓ Repeat with another layer of sliced potatoes and the remaining cheese mixture.

✓ Cover the baking dish with aluminum foil and bake for 25 minutes.

✓ Remove the foil, sprinkle chives over the top (if using), and continue baking until the crust is golden brown and the filling is bubbly (about 20-25 more minutes).

✓ Let the pie rest for a few minutes before slicing to allow the filling to set.