Peel the potatoes and grate them, put them in a large bowl and pour on the cold water to prevent darkening. Chop the onions finely and mix them in a deep bowl with the spearmint, salt and oil.
Knead a soft dough from the flour, water and pinch of salt. Divide the dough into 2, roll it out into 2 thin sheets, depending on the size of the tray you'll be baking the patatnik in. Once the sheets are rolled out, drain the potatoes well.
Place the drained potatoes in the bowl with the onions and spices and stir. Dry the oven dish and smear it generously with oil, then place 1 sheet, pour in all of the potato mixture, distribute evenly along it and place the 2nd sheet.
Smear the sheet with oil on top so that its crust ends up crunchy. Put to bake in a preheated 212°F (100 °C) oven. After you've placed the tray in the oven, increase the temperature to 302°F (150 °C) for 30 min., then to 428°F (220 °C) for 10 min. (Baking time and temperature vary depending on oven).