# What You Need:
→ Potatoes
01 - 4.5 pounds russet or Yukon Gold potatoes, peeled and thinly sliced
→ Cheese Sauce
02 - 4 tablespoons unsalted butter
03 - 1 medium yellow onion, finely diced
04 - 4 garlic cloves, minced
05 - 1/4 cup all-purpose flour
06 - 3 cups whole milk, warmed
07 - 1 cup heavy cream
08 - 2 1/2 cups sharp cheddar cheese, shredded, divided
09 - 1 cup Gruyère cheese, shredded
10 - 1 teaspoon salt
11 - 1/2 teaspoon black pepper
12 - 1/4 teaspoon cayenne pepper (optional)
13 - 1/2 teaspoon dry mustard powder (optional)
→ Garnish
14 - 2 tablespoons fresh chives or parsley, chopped (optional)
# How To Make:
01 - Preheat oven to 350°F. Grease a 9x13 inch baking dish with butter or nonstick spray.
02 - Melt butter in a large saucepan over medium heat. Add diced onion and sauté for about 5 minutes until soft and translucent. Stir in minced garlic and cook for 1 minute until fragrant.
03 - Sprinkle flour over the onion mixture and stir constantly for 2 minutes to form a roux.
04 - Gradually whisk in warmed milk and heavy cream. Continue cooking, whisking frequently, until the sauce thickens and bubbles, about 5 to 6 minutes.
05 - Reduce heat to low. Stir in 2 cups cheddar and all Gruyère cheese until melted and smooth. Season with salt, black pepper, cayenne pepper, and dry mustard powder as desired.
06 - Arrange half of the sliced potatoes evenly in the prepared baking dish. Pour half of the cheese sauce over the potatoes, spreading evenly.
07 - Layer the remaining potatoes on top, then cover with the remaining cheese sauce.
08 - Sprinkle the remaining 1/2 cup of shredded cheddar over the top.
09 - Cover the dish with aluminum foil and bake for 45 minutes.
10 - Remove the foil and continue baking for 30 to 35 minutes, until potatoes are tender and the top is golden and bubbly.
11 - Allow to rest for 10 to 15 minutes before garnishing with chopped chives or parsley and serving.
