Preheat the oven to 400 degrees. Wash and peel your potatoes and then slice them into rounds about 1/8 of an inch thick. Line them in your baking dish and gently fan open some of the potatoes so that sauce can get between the layers.
4 lbs Russet or Yukon Gold Potatoes
In a medium saucepan, melt your butter on medium heat. Then, add onions and sauté until golden brown.
4 Tbsp Butter, 1/2 Cup Yellow Onion
Once your onions are browned, add in your flour. Stir quickly and make sure it is thoroughly mixed in. Turn the heat down to low.
1/4 Cup Flour
Next, add your chicken broth in slowly, mixing together well. The sauce will be thick. Then, add your heavy cream in slowly.
2 Cups Chicken Broth, 2 Cups Heavy Cream
Once all the cream is incorporated, add in your seasonings.
2 Tbsp Fresh Thyme, 1/4 tsp Pepper, 3/4 tsp Garlic Powder, 1 pinch Salt
Pour the finished cream sauce over the sliced potatoes, making sure to cover all of them.
Sprinkle Cheddar and Parmesan evenly on top.
2 1/4 Cups Shredded Cheddar, 1/2 Cup Parmesan Cheese
Wrap tightly in aluminum foil and bake in the preheated oven for 40 minutes covered.
Remove the foil and bake for another 30 minutes, or until the potatoes are fork tender. Serve Warm