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Export 12 ingredients for grocery delivery
Preheat the oven to 375 degrees F. Spray a 2 quart square baking dish (or an 8x12 is good) with nonstick cooking spray or grease with butter or oil.
Place sliced potatoes in three tight slanted rows as indicated in the photos. You don’t want them to be overly tight, make sure you leave enough space so the potato slices are slightly slanted. Add in onion slices in between the rows and on top of potatoes. Basically wherever you can fit them.
Next make your sauce: add in 2 tablespoons of butter and place over medium heat. Once butter is melted, whisk in a little bit of the flour and then slowly add in milk, a little bit at a time, alternating with the flour and vigorously whisking away any lumps. Bring mixture to a simmer; it should be a similar consistency to creamy gravy.
Turn heat to low and stir in shredded cheddar cheese, garlic powder, salt and pepper. Taste and add more salt and pepper, if necessary.
Pour the sauce evenly over the potatoes and onions to cover them. Don’t worry, the sauce will get nice and creamy while baking.
Cover the pan with foil, then bake for 45 minutes.
After 45 minutes, remove foil, sprinkle top of potatoes with 1/2 cup shredded gruyere (or more cheddar) and ¼ cup parmesan. Return to the oven and bake for 30-45 more minutes or until potatoes are done and nice and golden brown on top. (Potatoes are done when the middle of the potatoes can be easily pierced with a fork.) Remove from the oven and garnish with fresh chopped parsley. Enjoy!