Make the dough: Into the bowl of a stand mixer, pour 1 cup of lukewarm water, then add the yeast and sugar and stir to combine. (It’s okay to leave a few clumps; the yeast will dissolve.) Cover the bowl with a tea towel or plastic wrap and set aside until the mixture becomes foamy and frothy, about 10 minutes.
In a large bowl, stir together the bread flour and salt, then add it to the yeast mixture and, using a silicone spatula, stir until the water is fully incorporated. Add ¾ cup of lukewarm water and continue stirring until a very shaggy dough forms, about 1 minute.
Place the bowl on the stand mixer and, using the hook attachment, mix on medium-low speed just until the dough is smooth, 3–4 minutes (it will be slightly sticky).
Lightly oil the large bowl from step 2, then transfer the dough into it, cover tightly with plastic wrap, and set aside in a warm place until nearly doubled in size, 45–60 minutes.
Position a rack in the lower third of the oven and place a pizza stone or steel on it. Preheat to 500°F.
Place a 16-by-10-inch sheet of parchment on a clean work surface and generously dust with flour. Using a bowl scraper, cut the dough in half right in the bowl. Scoop out one half of the dough and place it on the floured parchment. Gently press and stretch the dough into a 9-inch log, dust the surface lightly with flour, cover loosely with plastic wrap, and set aside. Place a second sheet of parchment on the work surface and repeat with the second half of the dough. Set aside until the loaves are very soft and puffed but not quite doubled in size, 20–25 minutes.
Meanwhile, make the topping: In a small bowl, whisk together the yogurt, salt, and 3 tablespoons of water. In a second small bowl, mix together the nigella and sesame seeds.
Uncover one of the loaves and, using your fingertips and adding flour as needed to avoid sticking, gently flatten and push the dough outward, pulling the sides apart as you go, to make a 14-by-8-inch rectangle. Using your pinky and ring fingers, press lengthwise channels into the dough, about 1 inch apart. (Push deeply, but don’t tear the dough.) Brush the surface with half of the glaze, then sprinkle with half of the nigella-sesame mixture.
Use the parchment to slide the glazed loaf onto an inverted baking sheet, then use the baking sheet to transfer onto the hot pizza stone. (You can leave the parchment underneath while it bakes.) Bake, rotating the loaf 180 degrees halfway through, until the surface and bottom are lightly but evenly browned, 10–12 minutes. Remove from the oven and cool on a wire rack while you repeat with the second loaf. Cool for 10 minutes, then cut crosswise into 2-inch slices and serve warm.