Focaccia with Rosemary and Onions

Directions

1. In the large bowl of an electric mixer fitted with the dough hook stir together the yeast, the sugar, and water and proof the yeast mixture for 5 minutes, or until it is foamy.
2. Stir in 4 ½ cups of the flour, the salt, and 2 tablespoons of the oil and knead the dough, scraping down the dough hook occasionally and adding as much of the remaining ½ cup flour as necessary to form a soft, slightly sticky dough, for 3 minutes.
3. Transfer the dough to a lightly oiled bowl, turn it to coat it with the oil, and let it rise, covered, in a warm place for 1 hour, or until it is double in bulk.
4. Preheat oven to 400 degrees F.
5. Press the dough with lightly oiled hands into a well-oiled 15 ½— by 10 ½-inch jelly-roll pan, and let it rise, covered loosely, for 30 minutes.
6. Meanwhile, saute the onion slices in 1 tablespoon olive oil until softened but not brown.
7. Dimple the dough with your fingertips, making ¼-inch-deep indentations, brush it with the remaining 1 tablespoon oil, and top it with the onion, salt, and rosemary.
8. Bake the focaccia in the bottom third of a preheated 400°F. oven for 35 to 45 minutes, or until it is golden and cooked through. Transfer the focaccia to a rack, let it cool for 10 minutes, and serve it, cut into squares, warm or at room temperature.