Resist the urge to add more flour to this dough. A slightly wet dough makes a crisp focaccia with a light texture. Starting the bread with a sponge and an overnight rising in the refrigerator helps the yeast develop slowly, so it's flavor will be more pronounced. But you can skip the overnight rise if it doesn't suit your schedule.
1 package dry yeast (about 2 1/4 teaspoons)
1 teaspoon honey
2 cups warm water (100° to 110°)
2 cups all-purpose flour
1 cup bread flour
1 cup whole wheat flour
4 teaspoons olive oil, divided
2 teaspoons sea salt
1 teaspoon olive oil
5 cups thinly sliced red onion
1 tablespoon chopped fresh rosemary
1/4 teaspoon sea salt
1/4 teaspoon crushed red pepper
How to Make It
To prepare the sponge, dissolve yeast and honey in warm water in a large bowl, and let stand 5 minutes. Lightly spoon all-purpose flour into dry measuring cups; level with a knife. Stir all-purpose flour into yeast mixture. Cover and chill overnight.
To prepare dough, stir yeast mixture with a spoon; let stand 30 minutes or until it begins to bubble. Combine yeast mixture, bread flour, whole wheat flour, 1 tablespoon oil, and 2 teaspoons sea salt in a large bowl. Beat with a mixer at medium speed for 15 minutes or until dough pulls away from sides of bowl. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size (dough will be wet).
Spread 1 teaspoon oil evenly over bottom of a 15 x 10-inch jelly roll pan. Pour dough into pan; let stand 5 minutes. Gently press dough to fill pan; let stand 30 minutes.
Preheat oven to 425°.
While the dough is rising, prepare topping by heating 1 teaspoon oil in a large nonstick skillet over medium heat. Add onion and rosemary, and cook 15 minutes or until browned. Arrange onion over top of the dough. Sprinkle with 1/4 teaspoon sea salt and crushed red pepper. Bake at 425° for 25 minutes or until golden brown. Cool 5 minutes.