01 -
Heat oven to 400°F and grease a 9x13-inch baking dish.
02 -
Melt butter in medium skillet over medium heat. Add flour and whisk 1 minute. Slowly pour in milk then broth, whisking constantly. Add garlic, thyme, salt, and pepper. Cook 2-3 minutes until sauce coats spoon. Remove from heat.
03 -
Spread half the potato slices in baking dish. Top with half the onion, sauce, and 1 cup cheese.
04 -
Add remaining potatoes, onion, sauce, and ½ cup cheese.
05 -
Cover with foil, bake 30 minutes. Uncover, bake 35-40 minutes until potatoes are tender and top is brown.
06 -
Let stand 20 minutes before serving.