To prepare a three onion pie with feta cheese robert cheese recipe, you will first need to prepare your pie crust. Preheat the oven to 375 degrees F (190 degrees C) and grease the bottom and sides of the pie pan with butter or olive oil. Cut the onions in half lengthwise and slice each half into 1/4-inch slices. Place the slices on the prepared pan and sprinkle with the salt and pepper. Bake in the preheated oven for 20 to 25 minutes or until the crust is golden brown.