This Italian flatbread is perfect for sandwiches like our Sauteed Mushroom, Prosciutto, and Taleggio Panini.
Sprinkle yeast and sugar over 1/2 cup warm water in a large bowl. Stir to dissolve, and let stand 10 minutes, until the mixture is foamy. Add remaining 2 cups warm water, 2 tablespoons oil, 2 cups flour, and salt; stir with a wooden spoon until smooth. Gradually add 4 cups flour, using your hands once mixture becomes too thick to stir. If dough is still too wet, add just enough of the remaining cup flour until it is no longer sticky.
Turn out dough onto a clean work surface; knead until smooth and elastic, about 10 minutes, using any remaining flour, if needed. Place dough in a lightly oiled bowl, and cover with plastic wrap. Let rise in a warm place until doubled in bulk, about 1 1/2 hours.
Preheat oven to 425 degrees. Transfer dough to an oiled 11-by-17-inch rimmed baking sheet; stretch to fill sheet, and press to form an even thickness. If dough shrinks back, cover with a damp towel, and let rest 10 minutes before proceeding. Once dough fills sheet, cover with a damp kitchen towel; let rise again until doubled in bulk, about 30 minutes.
Using your fingers, make dimplelike indentations all over surface of dough. Drizzle with remaining 2 tablespoons oil; spread with your hands to coat evenly.
Place baking sheet in oven. Toss a cup of ice cubes into bottom of oven to create steam (if using a gas oven, place a small baking dish of ice water on oven floor). Bake until golden brown, about 30 minutes, rotating pan halfway through; avoid opening oven any more than necessary or the steam will escape. Transfer focaccia to a wire rack to cool.
This bread is best eaten the day it is made but will stay fresh for up to four days when wrapped well in plastic and stored in the refrigerator.