Grate potatoes and onions alternately using a box grater or a food processor with the shredding disk.
Transfer the mixture onto a thin, clean dish towel; gather the corners and holding it over a large bowl; squeeze out as much liquid as possible.
Don’t discard the liquid yet and let it sit for 2 minutes to let the starch settle to the bottom. A couple minutes later, discard the liquid but keep the starch deposit.
Transfer the potato mixture to a bowl and add beaten eggs, matzo meal, flour, salt, freshly ground black pepper and parsley. Stir until well combined and set aside.
In a large cast iron pan over medium-high, heat oil and duck fat until it reaches 350ºF.
Meanwhile, using a ¼ cup measuring cup, portion the potato mixture and flatten them down to about ½-inch thick.
When the oil reaches the right temperature, gently add the latkes making sure not to overcrowd the pan. Fry them until golden brown, about 3 minutes per side flipping them carefully.
When cooked, transfer them to a wire rack sitting on a baking sheet lined with paper towels. If making a large batch, keep latkes in the oven at 170ºF.
Serve with apple sauce and crème fraîche