Make the dough: Put the bread flour, whole wheat flour, water, olive oil, sugar, salt, and yeast in the bowl of a stand mixer fitted with a dough hook. Blend the ingredients for a minute or so on low speed and then knead for 15 minutes, or until a smooth dough forms.

Grease a large bowl with olive oil and transfer the dough to the bowl, turning it to coat it with oil. Cover the bowl with plastic wrap and let the dough rest in a warm place until doubled in size, about 1½ hours. (Alternatively, let the dough rise in the refrigerator until doubled, about 24 hours.)

While the dough rises, make the filling: put the spinach in a large bowl and sprinkle with the salt. Massage the spinach with your hands for 1 to 2 minutes, until it wilts and releases some liquid, then set aside for 20 minutes.

Squeeze out the excess liquid from the spinach, transfer it to a cutting board, and chop it roughly. Return the spinach to the bowl and add the feta, string cheese, and halloumi. Stir until the mixture is well combined and set aside.

When the dough is ready, transfer it to a lightly oiled surface and divide it into about 20 equal pieces, about 3 ounces each.

Arrange the pieces of dough 1 inch apart on an oiled baking sheet, cover with plastic wrap, and let rest for 20 minutes.

Preheat the oven to 475°F and place a pizza stone on the middle rack of the oven, if you have one. Alternatively, cover the underside of a baking sheet with aluminum foil and place it on the middle rack of the oven, upside down.

Assemble the kadeh: lightly flour the work surface. Roll 1 piece of dough into a 6- to 7-inch round. Put 2 heaping tablespoons of the spinach filling onto the lower half of the dough, leaving a ½-inch border around the edges. Brush a bit of water along the edges of the dough. Fold the dough over the filling, creating a half-moon shape, and seal the edges, pressing down firmly with your fingers. Crimp the edges decoratively by pressing dimples into them with your index finger, then prick the dough all over with a fork. Repeat with another 2 or 3 pieces of dough.

Put the first few kadeh on the preheated pizza stone or baking pan and continue making more kadeh a few time, rolling and stuffing them as the first batches bake. Bake the kadeh until golden and baked through, 8 to 10 minutes (you may need to test the baking time by cutting into one of the breads). Take the kadeh out of the oven and place on a wire rack, then immediately brush the tops of the kadeh with olive oil and sprinkle with nigella seeds. Cover the baked kadeh loosely with a cloth to keep warm as you bake the rest. Repeat until you've assembled and baked all the kadeh. Serve warm.