Preheat oven to 400 degrees F. While the oven is preheating, par-boil the sliced potatoes for approximately 5 minutes. Drain then follow the instructions.
Lightly spray a 9x13 baking dish with cooking spray. Add the potatoes to the dish and season with salt and pepper. Combine the cheeses in a bowl.
Combine the milk and cornstarch in a medium saucepan and bring to a boil. Boil, while whisking continuously for about 1 minute. Turn off the heat and add half the cheese and the yogurt. Stir in the thyme and season lightly with salt and pepper. Pour the sauce over the potatoes.
Cover and bake for 30 minutes, until bubbly. Uncover, sprinkle with remaining cheese and bake for 15 more minutes, until potatoes are tender when pierced with a knife and cheese is browned and bubbly.