Step 1

Preheat oven to 375° F. Grease a 9x13-inch baking dish with cooking spray.

Step 2

In a medium saucepan, melt the butter over medium heat. Add the garlic and flour and cook, stirring, for 30 seconds.

Step 3

Whisk in the milk, broth, salt and pepper and heat, stirring occasionally, until the mixture just barely comes to a simmer. Then remove the pan from the heat and stir in the shredded cheddar and grated Parmesan cheese.

Step 4

Lay half of the sliced potatoes in an even layer in the baking dish, so that the slices overlap (it doesn't have to be perfect). Lay all of the sliced onions evenly over the top of the potatoes. Pour half of the cheese sauce evenly over. Then layer the remaining potatoes and pour the remaining cheese sauce evenly over the top.

Step 5

Cover the dish with foil and bake for 45 minutes. Then uncover and bake for 35-45 minutes more, until potatoes are tender and the casserole is golden brown on top. Let casserole rest for 5 minutes. Then sprinkle on fresh thyme or rosemary, if desired, and serve.