Boil the potatoes in salted water until fork-tender, about 15-20 minutes. Drain, peel, and set aside to cool. Meanwhile, grate the onion using a box grater. Place the grated onion in a cheesecloth or in your hands and squeeze tightly to remove all excess liquid. Discard the liquid.

In a large mixing bowl, mash the cooled potatoes until smooth. Add the ground beef, squeezed grated onion, beaten egg, turmeric, salt, and black pepper. Use your hands to gently mix until all ingredients are well combined. Do not overmix.

Spread the breadcrumbs on a large plate. Take about 1/4 cup of the meat mixture and form it into a flat oval patty, about 1/2-inch thick. Gently press both sides of the patty into the breadcrumbs to coat completely. Repeat with the remaining mixture.

Heat the vegetable oil in a large skillet over medium-high heat. When the oil is shimmering, carefully add the patties to the pan, ensuring not to overcrowd it. Fry in batches for 4-6 minutes per side, until deep golden brown and cooked through. Transfer the cooked kotlet to a wire rack to drain.

Serve hot or at room temperature with flatbread, tomatoes, and Persian pickles.