Preheat oven to 350˚F. Grease a 9"x13" baking dish with butter or cooking spray.
Using a mandolin or sharp knife, cut the entire onion and potatoes separately a 1/8" thin. Set aside.
In a skillet or Dutch oven melt butter and add onion and saute several minutes over medium heat. Add garlic, salt, pepper and saute until onion is softened, about 3 minutes. Add flour and cook for 1-2 minutes, stirring to mix thoroughly and cook flour.
Stir in the milk, chicken stock, Ranch Seasoning and bring to a boil. Lower heat and simmer. Continue to stir with spoon or whisk. Sauce will thicken after 2-3 minutes.
Layer 1/3 of the potatoes in the bottom of a 9"x13" baking dish and pour 1/3 of the cream sauce sauce over top. Spread evenly using spatula. Repeat layers 2 more times ending with cream sauce on top. Cover with aluminum foil and bake for 45 minutes.
Uncover and bake for an additional 45 minutes or until golden brown and potatoes are tender. Turn on broiler 3-4 minutes for golden top.
Allow to rest for 15 minutes before cutting/serving to cool and firm up.
Video
Notes
I use a Russet or Golden Yukon potatoes.
Feel free to use heavy cream for a richer, thicker sauce or Half and Half.