Preheat oven to 375 degrees and grease a 9 x 13 inch baking dish.

Add the potatoes to a large pot of cold water salted with the tablespoon of Kosher salt. Bring the mixture to a boil and cook for 7-10 minutes or until fork tender, then drain and add to the baking dish.

Bring the chicken broth and butter to a boil in a medium saucepan.

In a measuring cup, whisk together the milk, flour, garlic powder, and onion powder. Pour the milk mixture into the boiling broth and continue to cook, whisking constantly, until the mixture is thickened and coats the back of a spoon. Lower the heat and season to taste with salt and pepper.

Stir in sour cream and 1 1/2 cups of the cheddar until melted.

Pour the sauce over the potatoes along with most of the green onions. Top with remaining 1 1/2 cups cheese.

Bake for 20 minutes or until cheese is browned and mixture is bubbly. Top with the remaining green onions before serving.