In a medium saucepan, melt the butter over medium heatand whisk in the flour cooking until golden. Whisk in the milk and continue to cook until slightly thickened.
Add the vegetable broth, nutmeg, garlic powder, salt and pepper and bring to a boil stirring constantly. Continue cooking until thickened. Remove from the heat and set aside.
In a small sauté pan coated with cooking spray, sauté the onions until softened. Add the garlic and cook for 1 minute. Remove from the heat and set aside.
In a large bowl, add the hash browns, béchamel sauce, onion mixture, cheese, and yogurt. Pour the mixture into a 13x9 inch baking dish coated with cooking spray and spread into an even layer.
In a small bowl, mix together the cornflakes and melted butter. Sprinkle the mixture evenly over the potatoes and bake for 40 minutes or until bubbly and golden brown. Serve immediately.