Preheat oven to 350 degrees and lightly coat a 9×13 baking dish with non-stick spray. Set aside.
Wash and dish your potatoes Add them to a large pot of salted water and bring to a boil. Once boiling, drain them and return them to the pot.
While the potatoes are cooking, in a large saucepan, combine the milk and cornstarch and whisk together. Over medium high heat, bring to a boil, whisking often, until it is bubbly and thick. It comes together quickly.
Remove from the heat and stir in the cheese, yogurt, onion and 1/2 teaspoon of salt and pepper until combined.
Pour the sauce over the pot of potatoes and stir to fully coat them.
Spread the coated potatoes into the prepared baking dish evenly.
Smash up the chips and sprinkle them evenly over the top of the potatoes.
Bake for about an hour, until the potatoes start to brown and the edges are bubbling. Let cool for about 20 minutes before serving.