Spray a lasagna style pan with non-stick cooking spray.
Melt butter in a medium skillet over medium heat. Add onions and garlic, saute for 5 minutes or until onion starts to soften.
Whisk in flour and cook an additional three minutes to cook out the raw taste of flour.
Slowly whisk in buttermilk and half and half. Continue stirring over medium heat until liquid starts to bubble slightly.
Place half the potatoes in the prepared lasagna pan.
Pour half the buttermilk sauce over the potatoes.
Arrange remaining potatoes on top and pour on the rest of the buttermilk sauce.
Cover and cook for 45 minutes.
Uncover and cook for an additional 45 minutes or until potatoes are browned.