Adjust one oven rack to lower-middle position and one to highest position. Preheat oven to 400 degrees F.
In a small bowl, toss together cheese, garlic and thyme; set aside.
Measure out 1/4 cup of cheese mixture and set aside.
In a medium saucepan heat butter over medium heat until it begins to foam. Whisk in flour and cook until bubbly, about 30 seconds. Whisking constantly, gradually add chicken stock. Increase heat to high, add bay leaf and bring to a boil. Reduce heat to low and simmer until mixture thickens, about 2 mintues. Remove from heat. Remove bay leaf and cover pan to keep warm.
Alternating yukon and russetts, arrange half of the potatoes in overlapping rows. Sprinkle layer with half the salt and pepper. Cover with half the cheese mixture. Repeat with a second layer. Pour sauce over all. Cover with foil and bake on lower rack for 45 minutes.
Remove potatoes from oven and sprinkle with remaining 1/4 cup cheese. Return potatoes to oven on highest rack and bake until cheese is golden.