Mix the dough

• In a large bowl, combine the sourdough starter, warm water and olive oil, mixing until the starter is mostly dissolved.
• Add the flour, then the salt, and mix until a shaggy dough forms with no dry pockets of flour.
• Scrape the dough off your hands and down the sides of the bowl.

Rest the dough

• Cover the bowl with a damp kitchen towel and let the dough rest on the counter.

Stretch and fold

• Perform a set of stretch and folds on the dough.
• Cover and rest for 30 minutes, then repeat one more set of stretch and folds.

Bulk fermentation

• Cover the dough and let it rise on the counter until fully doubled in size, ideally overnight.

Prepare the baking dish

• Grease a 9x13 baking dish with butter, including the sides.
• Add olive oil to coat the bottom of the dish.
• Pour the dough into the dish.

Proof the dough

• Let the dough rest on the counter until it spreads to fill the dish and rises a little more.

Roast the garlic

• Place the garlic cloves in the middle of a piece of foil, drizzle with olive oil and sprinkle with salt.
• Fold and seal the foil like an envelope and roast on the middle rack until golden and soft.
• Chop the rosemary leaves.

Top and dimple

• Drizzle the dough with olive oil and sprinkle with flaky salt and the chopped rosemary.
• Wet your fingertips and dimple the dough all over, stretching it to the corners if needed.
• Push the roasted garlic cloves down into the dimples.

Bake

• Bake until golden brown.
• Remove from the oven and let cool slightly before cutting.