• 4½ teaspoons or 2 packages active dry yeast
• 2 teaspoons sugar
• 2 cups lukewarm water
• 3 tablespoons extra-virgin olive oil, plus more for skillet
• ¾ teaspoon cayenne pepper
• 5½ cups unbleached all-purpose flour, plus more for dusting
• ½ cup bread flour
• ¼ cup semolina flour
• Salt
• 3 tablespoons extra-virgin olive oil, plus more for pan
• 10 ounces spring onions or scallions (about 2 bunches), sliced
• 2 teaspoons sugar
• 1½ teaspoons coarse salt

In the bowl of an electric mixer fitted with the dough hook, dissolve the yeast and sugar in the water. Add 2 tablespoons of the oil and cayenne. In another bowl, whisk together the all-purpose flour, bread flour, semolina flour and 4 teaspoons salt. With the machine running on medium-low speed, add about half of the flour mixture to the liquid; beat until well blended.

Continue adding the flour mixture in ½ cupfuls, beating well after each addition, until the dough is smooth and slightly sticky, 8 to 10 minutes. Turn out dough onto a lightly floured surface and knead briefly to form a ball. Place in a lightly oiled bowl, cover with a damp cloth or a piece of lightly greased plastic wrap and let rise until doubled in volume, 30 to 45 minutes. 

Meanwhile, generously coat a skillet with oil and heat over medium heat. Add the onions, sugar, and 1 teaspoon salt; cook, stirring occasionally, until softened, about 5 minutes.

Place two racks in the upper and lower third of the oven and preheat to 400°. Using olive oil, lightly grease two 11-x-17-inch rimmed baking sheets. Punch down the dough and divide in half.  With your fingertips, push the dough out toward the edges of the sheet. In a small bowl, combine remaining tablespoon oil with 2 teaspoons water; brush over dough. Sprinkle with the coarse salt. Let rise, uncovered, for 30 minutes.

Bake, rotating sheets and switching top sheet to bottom rack halfway through, until light brown, 10 to 15 minutes. Immediately slide the focaccia onto wire racks and let cool slightly. Slice and serve warm or at room temperature, topped with the onions.