Prepare the Potato-Onion Filling. Preheat the oven to 250 F. Place a 4- to 5-inch non-stick skillet over medium low heat until a shred of the cheese begins to sizzle 2 to 3 seconds after it hits the pan. Scatter one-eighth (about 1/3 cup) of the cheese in an even layer over the bottom of the skillet. Arrange one-fourth of the potato-onion filling over the cheese and press it very gently into an even layer. Sprinkle another 1/3 cup of the cheese over the filling in an even layer. Let the bottom layer of the cheese cook without disturbing it or moving the pan until the fat which separates from the cheese begins to bubble around the edges, about 3 minutes. At this point shake the skillet gently to free the cheese crisp from the bottom of the pan. If it sticks, let it cook a minute or two, then try again. If at that time, the crisp is still sticking in places, carefully work a heatproof rubber spatula under the crisp to free it. In about 6 to 7 minutes total cooking time the underside of the crisp should be an even golden brown and the crisp should slide very easily in the pan. Slide the crisp onto a small plate then invert the crisp back into the skillet. Cook the other side as you did the first. Slide the crisp out onto a baking sheet and keep it warm in the oven while cooking the remaining fricos. Pat dry with paper towel, cut in half, top with dressed greens if you like, and serve. To cook filled fricos on a griddle: Heat a cast-iron or other heavy, stick-resistant griddle over medium-low heat until a shred of the cheese begins to sizzle 2 to 3 seconds after it hits the surface of the griddle. Using one-eighth of the cheese for each, form as many circles about 4 inches in diameter as will fit on the griddle without touching. (If this is your first time making filled fricos, you may want to make six slightly smaller fricos on the griddle; they will be easier to flip.) Fill and cook as described above, simply flipping the crisps with a metal spatula when the underside is golden. Filled fricos cooked on a griddle will require about 12 minutes total cooking time.