Wrap the 1 head of garlic in aluminum foil and roast at 350°F (175°C) until soft, about 60 minutes.

Combine the roasted garlic, 3 cups pitted Castelvetrano olives, rosemary leaves, thyme leaves, lemon zest, 3 tablespoons olive oil, and optional red pepper flakes in a mixing bowl.

Lightly crush the olive mixture with a potato masher, then set it aside.

Mix the 4 1/2 cups bread flour with 2 cups warm water in a large mixing bowl until no dry flour remains and a shaggy dough forms.

Cover the shaggy dough with a damp kitchen towel and rest it for 30 minutes.

Using a wet hand, work 1 3/4 teaspoons instant yeast and the optional 1/4 cup sourdough starter into the rested dough.

Add 1 tablespoon kosher salt and 2 tablespoons olive oil, then fold and squeeze the dough until both are absorbed.

Cover the dough with the damp kitchen towel and rest it for 30 minutes.

Add the prepared olive mixture to the dough and fold the outer edges toward the center until the filling is enclosed.

Cover the dough and rest it for 30 minutes.

Fold the dough edges toward the center once more to strengthen the dough.

Cover the dough tightly and refrigerate it for 1080 to 1440 minutes.

Turn the chilled dough onto a lightly floured work surface and fold its edges toward the center.

Flip the dough seam-side down, shape it into a round, and rest it for 15 minutes.

Tighten the dough into a smooth round and place it on a sheet of parchment paper.

Cover the dough loosely and let it rise for 45 to 60 minutes, until light and bubbly.

Preheat the Dutch oven in the oven at 450°F (230°C) for 30 minutes.

Score the risen dough with a bread lame and transfer it with the parchment into the hot Dutch oven.

Add a few optional ice cubes beside the parchment, then cover the Dutch oven with its lid.

Bake the covered loaf at 450°F (230°C) for 30 minutes.

Remove the lid and bake for 20 to 25 minutes, until deeply golden and the bread center reaches at least 208°F (98°C).

Transfer the baked loaf to a wire cooling rack and cool it completely before slicing.