Set a rack in the middle of your oven and put your sheet pan on the rack. Preheat the oven to 425 degrees.
Cut potatoes into ½-1 inch pieces (1-inch pieces will take a little longer to cook). Use a paper towel to blot off as much moisture as you can.
Toss the cut potatoes in a bowl with the kosher salt, and let them sit for 5-10 minutes.
Combine the granulated garlic, onion powder, paprika, cornstarch or brown rice flour, and pepper.
Blot the potatoes off with a paper towel one more time, then toss them with the olive oil. Add the seasoning mix and toss to coat.
Take the sheet pan out of the oven and close the oven to preserve heat. Spray the pan with a very light coating of cooking spray.
Carefully spread potatoes out on the pan and shake the pan to spread them out. Use tongs to make sure every piece has a cut side in contact with the pan. Return the pan to the oven and roast for 15 minutes.
Take the pan out of the oven and close the oven to preserve the heat. Stir the potatoes well, using tongs to turn any over that don’t look browned and crispy on the top. Try to make sure every piece of potato gets turned…it’s worth the little bit of extra effort!
Return the pan to the oven and roast for 12-15 more minutes, until the potatoes are crispy on the outside but a fork goes easily through the middle.
Transfer the potatoes to a warmed bowl and, if desired, top with crumbled feta and chopped fresh parsley.