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1
Adjust one oven rack to lower-middle position and second oven rack to highest position; heat oven to 400 degrees.
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2
Toss together cheese, garlic, and thyme in small bowl; set aside.
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3
Heat butter in medium saucepan over medium heat until foaming; whisk in flour and cook until golden and bubbly, about 30 seconds.
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4
Whisking constantly, gradually add chicken stock. Increase heat to high, add bay leaf, and bring to boil.
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5
Reduce heat to medium and simmer, stirring occasionally, until mixture thickens, about 2 minutes. Off heat, remove and discard bay leaf; cover to keep sauce warm.
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6
Alternating Yukon Gold and russet slices, arrange a third of potatoes in snug overlapping rows in 9x13 baking dish, Sprinkle potato layer evenly with 1/4 teaspoon salt, 1/4 teaspoon pepper, and a third of cheese mixture. Make a second layer with same amount of potatoes, salt, pepper, and cheese. Make a third layer with same amount of potatoes, salt, and pepper, but reserve remaining cheese. Pour warm sauce over potatoes, tilting pan side to side to distribute evenly.
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7
Cover with foil and bake on lower-middle rack 45 minutes.
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8
Remove foil and continue to bake until potatoes are tender, about 45 minutes longer.
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9
Remove potatoes from oven and sprinkle with remaining cheese; bake on highest rack until cheese is golden brown, about 5 minutes.
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10
Cool 10 minutes and serve.
