2 tablespoons butter plus more for baking dish
1 cup finely chopped leeks (if needed, sweet onion may be substituted)
2 cloves garlic, grated/minced
1 teaspoon dried thyme
1 cup whole milk
1 cup heavy cream
2 teaspoons kosher salt
1/2 teaspoon ground black pepper
2 1/2 pounds yellow/golden potatoes, peeled and sliced thin
Preheat the oven to 350-degrees F. Coat an 8×8-inch (or approximate) baking dish with butter.
Heat a medium saucepan over medium-high heat. Add butter. When butter has melted, add leeks; cook, stirring for 2 minutes.
Add garlic, thyme, continue to cook, stirring for 30 seconds.
Stir in milk, heavy cream, salt, and pepper. Bring to a boil, lower heat to a simmer, cook, stirring occasionally, until mixture has slightly thickened, about 5 minutes.
Add potatoes; stir to coat potatoes. Transfer mixture to prepared dish, spreading potatoes evenly in dish.
Bake until potatoes are done, lightly brown and bubbly, about 1 hour. If time permits, baste potatoes with sauce halfway through cooking. Let cool slightly before serving.