• Preheat oven to 375°F.
• Spray a 9×13-inch baking dish with cooking spray. Set aside.

Make the cheese sauce:

• In a medium saucepan over medium heat, melt the butter.
• Whisk in the flour, salt, and pepper. Cook for 1 minute.
• Slowly drizzle in the milk, whisking constantly.
• Continue cooking until the mixture thickens, 5-8 minutes, whisking frequently.
• Remove from the heat. Stir in 1 cup of cheese.
• Continue stirring until the cheese is melted and smooth, about 30 to 60 seconds.
• Taste and adjust the seasoning if necessary. Set aside.

Assemble and bake:

• Carefully set the potato slices in the bottom of the prepared baking dish (trying to keep the shape of the potato together), and fan them out in a single layer.
• Top the potatoes with the cheese sauce and the remaining cup of shredded cheese.
• Cover with aluminum foil and bake for 50 minutes.
• After 50 minutes, remove the foil and continue to bake for an additional 10 minutes, or until the potatoes are fork-tender.

While the potatoes are baking, prepare the fried onions:

• Line a large baking sheet with two layers of paper towels. Set aside.
• To a large saucepan over high heat, add the oil. Heat to 350°F.
• Working in batches, add about 1 cup of onions to the hot oil.
• Cook until the onions turn pale golden brown, about 2-5 minutes.
• Using a fine mesh strainer or slotted spoon, carefully remove the fried onions from the hot oil. Place them in an even layer onto the prepared baking sheet.
• Sprinkle the fried onions with salt.
• Set aside and repeat with the remaining onions, making sure the oil temperature is at 350°F before each batch and sprinkling each batch with salt immediately after frying.

Finish and serve:

• Sprinkle the fried onions evenly over the top of the scalloped potatoes.
• Serve hot, garnished with parsley.