Preheat oven to 400°F. Grease a 9 x 13-inch baking dish with gluten-free cooking spray.
In a medium saucepan, melt the butter over medium-high heat.
4 tablespoon Butter
Add the milk to the pan and whisk until combined with the butter.
2 cups Milk
Whisk the gluten-free flour into the milk mixture.
¼ cup Gluten-free all-purpose flour
Add the salt, pepper, onion powder, garlic powder, and dried thyme to the cream sauce, stirring until combined.
1 ½ teaspoon Salt, ¼ teaspoon Black pepper, 1 tablespoon Onion powder, 1 tablespoon Garlic powder, 1 teaspoon Dried thyme
Add the broth to the pan, and whisk until combined. Cook for 1-2 minutes until the sauce just barely begins to simmer around the edges of the pan and thickens. Remove from the heat once the sauce has thickened.
1 cup Gluten-free chicken broth or gluten-free vegetable broth
Spread half of the sliced potatoes in an even layer on the bottom of the greased pan.
4 pounds Yukon Gold Potatoes
Top evenly with half of the sauce.
Sprinkle the Parmesan cheese and 1 cup of the shredded cheddar cheese over the top of the first layer of potatoes and sauce.
½ cup Optional: Shredded Parmesan cheese, 2 cups Shredded cheddar cheese
Top evenly with the remaining sliced potatoes, the other half of the sauce, and the remaining 1 cup of cheddar cheese.
Cover the baking dish with aluminum foil, and bake for 30 minutes.
Remove the foil and bake uncovered for another 30 minutes, or until the potatoes are cooked through.
Allow the potatoes to cool for a few minutes before serving. Serve warm.