Achieve pillowy perfection with this sourdough focaccia.
To make caramelized onions, halve onions then slice crosswise into ¼”-thick half moons using a Chef Knife. Add sliced onions and a pinch of salt to a dry Stainless Clad Saute Pan and heat over high for 2–3 minutes, stirring occasionally. Reduce heat to medium-low and cover pan. Allow to cook, stirring every few minutes to make sure they don’t burn, until golden brown all the way through, 15–20 minutes. Transfer along with juices to a medium bowl and let cool completely to room temperature. Once cooled, transfer to a strainer set over a bowl and allow to drain. Reserve onions and juices.
To make the focaccia, add bread and whole wheat flours, 1655g warm water, and starter into a large bowl. Mix until all the flour is hydrated, about 4 minutes. Let the dough rest uncovered for 20 minutes.
After resting, add salt and 50g water to a small bowl and stir to combine. Add the salt water mixture into the rested dough and knead until just combined, then cover the bowl with a damp towel.
After 30 minutes, stretch and fold the dough by pulling the dough away from the sides of the bowl and lifting and stretching about half of the dough up above the bowl (do not allow dough to tear), then folding it towards you over the rest of the dough in the bowl. Repeat 2 more times for a total of 3 times. Keep a bowl of water on the side to wet hands to keep dough from sticking to fingers. Do not add additional flour. Cover and let rest 30 minutes. Next, pour 75g olive oil over top of dough. Pull the dough from the sides of the bowl and allow oil to drip under the dough and coat the bottom of the bowl, then stretch and fold 3 more times. Cover and let rest for 30 additional minutes. Measure 2 cups drained caramelized onions (reserve the remaining onion juices for baking). Stretch and fold dough again, 3 more times, adding a third of the onions each time. Let rest, covered, 30 minutes more. Sprinkle the chives over top of dough, then stretch and fold a final 3 times. You should have stretched and folded 4 rounds total—every 30 minutes over the course of 2 hours.
After the final stretch and fold, place the bowl in a warm area and allow the dough to rise for an additional 30 minutes. Then cover and rest the dough 12–24 hours in the fridge.
Divide ½ cup olive oil between 2 Baking Slabs or Sheet Pans and brush to coat evenly. Remove the chilled dough from the bowl and divide it in half, placing each half on a prepared Slab. Stretch the dough out to meet the edges of the Slab, then let it rest. Stretch the dough out to fit the Slab 4 times total—every 30 minutes for 2 hours. Drizzle remaining ½ cup olive oil evenly over the 2 doughs.
Preheat the oven to 450F. If using a Baking Slab, place on top of a Sheet Pan in the oven. If using a Sheet Pan, place another Sheet Pan or a large piece of foil on the bottom rack to catch any oil that drips. Bake 15 minutes, remove from oven and brush focaccia with reserved caramelized onion juices. Return to oven and bake until the top has nicely browned, about 15 minutes more.
Sprinkle with flaky salt and allow focaccia to cool on the Slab. Once focaccia has cooled to room temperature, slice each loaf into 12 pieces and serve. Store focaccia wrapped tightly in plastic for up to 2 days at room temperature, or up to 3 months in the freezer.