Preheat your oven to 400 degrees and spray a 9x13-inch baking dish with non-stick cooking spray.
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk for one minute, then slowly add the milk followed by the chicken broth, whisking continuously. Add the garlic, za’atar, kosher salt, ground pepper and cook, whisking often, for 2 to 3 minutes, or until the sauce lightly coats the back of a spoon. Remove from the heat.
Layer half of the potatoes at the bottom of the prepared baking dish. Top with half the sliced onion, then half the sauce and one cup of the shredded cheese. Repeat the layers with the remaining potatoes, onion, sauce, and ½ cup cheese.
Cover the dish with foil and bake for 30 minutes, then uncover and bake for 35 to 40 minutes, or until the potatoes are tender and the cheese on top is browned. Let stand at room temperature for 20 minutes before serving and garnishing with oregano, feta, and olives if desired.