In a skillet, saute chopped onion in butter until soft and tender.
Add minced garlic to the pan and cook until fragrant.
Whisk the flour into the pan and cook for a minute to cook off the raw flour flavor.
Whisk in the chicken broth and milk while whisking to break up the flour clumps. Season with salt and pepper to taste.
Bring the mixture to a simmer, then remove the pan from the heat and allow it to cool.
Use a mandolin to slice the potatoes into ⅛” rings.
Into a 9x13” baking dish, arrange half of the potato slices flat on top of each other. Season the potatoes with salt and pepper and top with half of the milk mixture.
Layer the remaining potato slices on top of each other. Season with salt and pepper, then pour the remaining milk mixture.
Cover the baking dish tightly with aluminium foil, and bake at 400°F for 45 minutes.
Uncover the dish and bake for an additional 15 minutes or until the potatoes are cooked through.