How to Make It
In the bowl of a standing mixer fitted with the dough hook, combine 2 cups of lukewarm water with the yeast and let stand for 5 minutes. Add the 4 cups of bread flour and 2 teaspoons of salt and mix at medium speed until a loose dough forms. Increase the speed to medium-high and mix until the dough is supple and smooth, about 6 minutes. Transfer the dough to a lightly floured work surface and knead for 1 minute. Transfer the dough to an oiled bowl, cover with plastic wrap and let stand in a draft-free spot until doubled in volume, about 1 hour.
Punch down the dough and form into 2 ovals. Transfer the ovals to an oiled baking sheet, cover with a sheet of oiled plastic wrap and let rise for 1 hour.
Meanwhile, in a small saucepan, combine the all-purpose flour with the sugar, 1 teaspoon of vegetable oil and 1/2 cup of water. Cook the flour paste over moderate heat, whisking, until thickened, about 2 minutes. Let the flour paste cool.
Preheat the oven to 450° and set a pizza stone on the lowest rack. Let the stone heat for at least 30 minutes. Generously sprinkle a pizza peel with semolina. Punch down 1 piece of the dough and transfer it to a lightly floured work surface. Press the dough to a 14-by-5-inch rectangle, then transfer to the peel; shake the peel lightly to make sure the dough doesn’t stick, adding more semolina if necessary. Using your fingers, press 5 deep lengthwise ridges into the dough. Rub about one-third of the flour paste over the surface and sprinkle with half of the nigella and sesame seeds. Slide the dough onto the hot stone and bake for about 18 minutes, until golden and risen. Repeat to make the second loaf (there will be some paste leftover). Serve warm.
Nutty, peppery nigella seeds (also called black onion seeds) are often used in Middle Eastern and Indian cooking. Look for them at specialty food stores.