Wash and scrub potatoes and set to the side. Using a mandolin, thinly slice (about ⅛-inch thick circles) the potatoes into a pot of cold water. Rinse the potatoes multiple times until the water runs clear. Fill the pot with enough water with only 1 inch of water over the top of potatoes. Sprinkle 1 tablespoon of salt over the potatoes then boil on medium-high for 15 minutes. Drain and rinse the potatoes in a colander and set to the side.
In the same pot, melt the unsalted butter over medium heat. Add the dried onion, flour and 1 teaspoon of salt. Continue mixing with a spatula until a rue is created. Using a whisk, gradually whisk in the milk until it simmers. Reduce the heat to low. Whisk in the Stonyfield Organic Plain Yogurt Double Cream until smooth. Add in the dried chives, parsley, black pepper and garlic powder. Whisk until incorporated then remove from the stove top and set to the side.
Spray a large baking dish with cooking spray. Pour a little bit of the sauce on the bottom of the baking dish. Then, layer half the potatoes in baking dish, pour half the sauce over the potatoes. Then repeat this step with the remaining potatoes and double cream sauce. Bake for 35 minutes or until bubbly and golden brown.
If you don't have a mandolin, pull out your knife skills! As long as your knife is sharp you should be able to make precise, even cuts.