2 T unsalted butter
1 small onion, minced
2 medium garlic cloves, minced
2 T flour
3 c heavy cream
1 c whole milk
1 t dried thyme
2 bay leaves
2 t salt
1/2 t ground black pepper
6 cups russet potatoes, cut into 1/8-inch-thick slices
1 c shredded cheddar cheese
Heat oven to 350 degrees. Meanwhile, melt butter in large pan over medium-high heat until foaming subsides, about 1 minute. Add onion and sauté until it turns soft and begins to brown, about 4 minutes. Add garlic and sauté until fragrant, about 30 seconds. Sprinkle flour over onions and garlic and cook a few minutes. Add cream, milk, thyme, bay leaves, salt, pepper, and potatoes and bring to simmer. Cover, adjusting heat as necessary to maintain light simmer, and cook until potatoes are almost tender (paring knife can be slipped into and out of center of potato slice with some resistance), about 15 minutes.
Remove and discard bay leaves. Transfer potato mixture to 3-quart gratin dish and sprinkle with cheese. Bake until cream has thickened and is bubbling around sides and top is golden brown, about 20 minutes. Cool for 5 minutes before serving.
