1. Heat the olive oil in a small sauté pan over medium-high heat. Add the onion and cook until it is softened and light in color, about 5 minutes. Remove from the heat and set aside to cool.
2. Combine the warm water, sugar, and yeast in the bowl of a standing mixer fitted with the dough hook. Let the mixture sit until it becomes foamy, about 5 minutes.
3. Add the flour, half of the herbs, the mashed potato, the 3 tablespoons extra-virgin olive oil, and the salt to the yeast mixture. Mix on low speed for 10 minutes, scraping
the dough and the hook about halfway through. The dough will come together and begin to pull away from the bowl. (Alternatively, you can mix the dough with a wooden spoon in a wide shallow bowl. Once you stir everything together, transfer the dough to a lightly floured work surface, and knead until smooth, about 10 minutes.)
4. Transfer the dough to a lightly floured work surface and knead it several times with floured hands to form it into a ball. Put the dough in a lightly oiled large mixing bowl, turn the dough so the entire surface is lightly oiled, cover the bowl with plastic wrap, and set it aside in a warm, draft-free place until the dough has doubled in size, about 1 hour. Punch down the dough, recover the bowl, and set it aside again to rise a second time, until doubled in size, about 1 hour.
5. Line an 11 3/4 x 7 1/2-inch rimmed baking sheet or a jelly roll pan with parchment paper, and dust it with cornmeal.
6. Transfer the dough to a lightly floured work surface and knead it several times with lightly floured hands to form it into a ball. Lightly dust a rolling pin with flour, and
roll the dough out to form a 12 x 16-inch rectangle; you may also use your hands to stretch the dough to manipulate it into the shape of the baking sheet. It will be pliable,
yet want to shrink. Transfer the dough to the prepared baking sheet and set it aside, covered, to rise a final time, about 1 hour.
6. Preheat the oven to 450°F.
7. Combine the remaining herbs with the remaining 1/2 cup extra-virgin olive oil in a small bowl.
8. Gently dimple the dough all over with your fingertips, leaving indentations about 1/2 inch deep. Brush the dough all over with half of the herb oil. Sprinkle the kosher salt
over the top. Bake for 10 minutes. Then rotate the baking sheet from back to front, and bake for 6 minutes longer, until golden. Remove the focaccia from the oven and brush
it with the remaining herb oil. Serve hot or at room temperature.
Note: You can use a variety of flavorings for focaccia. Instead of onion and herbs (or along with them) you can use olives, cheese, pesto, sun-dried tomatoes, and so on. You can always adjust the wetness of the dough (depending on its
flavoring) by adding more flour as needed.
Yield: 10 to 12 servings