Three-Cheese Au Gratin Potatoes

0 Preheat the oven to 375 degrees F. Spray an 8x12 baking dish with nonstick cooking spray or grease with oil.
1 Place sliced potatoes in 3-4 tight rows; making sure to leave enough space so the potato slices are slightly slanted. Add in onion slices in between the rows and on top of potatoes. Set aside.
2 To make the sauce: add in 4 tablespoons of butter, in a medium saucepan, and place over medium heat. Once butter is melted, whisk in the milk and flour milk, a little bit at a time, vigorously whisking away any lumps. Bring mixture to a simmer; it should be a similar consistency to creamy gravy.
3 Turn heat to low and stir in the seasonings and shredded cheeses. Taste and add more salt and pepper, if necessary.
4 Pour the sauce evenly over the potatoes and onions to cover them with foil. Bake for 45 minutes.
5 After 45 minutes, remove the foil, sprinkle the top of potatoes with the remaining cheeses and return to the oven and bake for 30 more minutes or until potatoes are done, and the top is nice and golden.
6 Remove from the oven and garnish with fresh parsley or green onions. Enjoy!