• Preheat oven to 350 degrees F. Grease a large 9"x13" casserole dish or baking dish with non-stick cooking spray. Set aside.

For the potatoes:

• Wash and scrub your potatoes then dry thoroughly. Thinly slice each potato into rounds approximately 1/8" thick (I recommend using a mandoline for this). If you are using russet potatoes, transfer to a large bowl filled with cold water to prevent them from turning a rusty-orange color. Set aside until ready to use.

To make the sauce:

• Melt the butter in a medium saucepan or Dutch oven set over medium heat. Add the diced onions and stir to combine. Cook the onions over medium to medium-low heat until softened, approximately 5 minutes. Add the garlic and continue to cook for an additional 1-2 minutes, stirring often, then, add the flour and continue to cook for 1-2 minutes, stirring continuously.
• Reduce heat to low. Slowly add the milk to the onions, whisking continuously to prevent any of the flour from clumping. Continue to slowly add the rest of the milk, then add the chicken broth.
• Increase heat to medium-high and bring to a low boil. Continue to cook, stirring as needed, for 1 minute. Remove from heat and stir in the heavy whipping cream (or half-and-half).

Assembly:

• Drain your potatoes and dry thoroughly. Place approximately 1/4 of the potato slices in the bottom of the baking dish and season with salt and pepper. Spoon approximately 1/4 of the cream sauce over the top.
• Repeat with an additional layer of potatoes until all the potatoes are used or you run out of space in your casserole dish (you should have room for at least 2-3 more layers), ending with a final layer of cream sauce over the top.
• Tent with foil and bake for 45 minutes. Uncover and bake for an additional 35-45 minutes, or until golden and bubbly around the edges.
• Allow to rest for 10 minutes before serving.