The bottom of börek (Turkish savory pastry), pide (Turkish flatbread), and tarts stays undercooked when enough heat does not reach the dough from below; the baking sheet is cold, the rack is too high, the sheet is thick and light-colored, or the filling is too wet. The solution is to increase the bottom heat: move the sheet to a lower rack, use a preheated sheet or baking stone, and reduce moisture in the filling.
The bottom of dough cooks differently from the top. The top surface receives heat from hot air and radiation from the upper heating element, while the bottom surface receives heat through contact with the baking sheet, by conduction. If the sheet is cold, this contact does not transfer heat at all during the first minutes; while the sheet heats up, the bottom of the dough steams and stays sticky. That is why the temperature of the baking sheet matters as much as the oven temperature for foods that should have a crisp bottom. When the top looks done, check the bottom separately.
The most effective solution is to preheat the baking sheet. Heat the empty sheet along with the oven; transfer the dough, on its parchment paper, onto this hot baking sheet. The bottom starts receiving heat from the first second and a crust forms right away. A baking stone or thick cast-iron sheet does the same job more effectively because it stores more heat. For Turkish flatbread, meat flatbread, and pizza, this method is the one adjustment that brings a home oven closer to a stone oven. Transfer the dough to the hot sheet with the help of a wide spatula or cutting board.
Rack and sheet choice also matter. A sheet on a lower rack receives stronger heat from the bottom heating element; dough baked on an upper rack often has a pale bottom. A dark, thin metal sheet transfers heat quickly and browns the bottom fast; thick, light-colored, or glass pans transfer heat slowly and the bottom cooks later. This is why silicone molds often leave the bottom pale; setting the mold on a metal sheet partly compensates. The choice of mold or sheet changes the result even when the recipe stays the same.
The filling is also part of the problem. Spinach that releases water, wet cheese, raw onion, or boiled potatoes can wet the underside of the dough from within. Cook the filling ahead of time to evaporate the water, drain it, and let it cool; a hot filling softens the dough with its steam. For pie and tart crusts, blind-baking the dough—baking it on its own before adding the filling—solves the same problem. A thin layer of semolina or bread crumbs sprinkled under the filling also holds moisture. A cold filling protects the edges of the dough, too.
A few details complete the result. Thick foil spread on the baking sheet reflects bottom heat and delays browning underneath; parchment paper does not have this effect. Fully preheat the oven; dough placed in a partially heated oven will never set underneath properly. Do not leave baked goods on the sheet after baking; move them to a wire rack, since the bottom of a pastry left on the sheet softens again in its own steam. If the bottom is still pale, finish the item for a few minutes directly on the lower rack. Next time, try the same recipe on a preheated sheet.