Preheat your oven to 400°F. Cut the top off the head of garlic, drizzle with olive oil, and wrap in aluminum foil. Roast in the oven for about 30-35 minutes, until soft and fragrant. Allow to cool, then squeeze the cloves out of their skins and mash them into a paste.

In a large mixing bowl, combine the flour, salt, and instant yeast. Add the warm water and the roasted garlic paste. Mix until a shaggy dough forms. If using fresh rosemary, fold it into the dough at this stage.

Cover the bowl with plastic wrap or a clean kitchen towel and let it rise at room temperature for 12 to 18 hours, until the surface is bubbly and the dough has doubled in size.

Once the dough has risen, turn it out onto a floured surface. Gently shape it into a ball, being careful not to deflate it too much. Dust the top with flour and cover it with a kitchen towel. Let it rest for 30 minutes.

While the dough is resting, place a Dutch oven (with the lid) in the oven and preheat to 450°F. Once the dough has rested, carefully remove the hot Dutch oven from the oven. Sprinkle the bottom with cornmeal or flour to prevent sticking, then place the dough inside. Cover with the lid and bake for 30 minutes.

After 30 minutes, remove the lid and bake for an additional 15-20 minutes, until the bread is golden brown and sounds hollow when tapped on the bottom. Remove from the Dutch oven and let cool on a wire rack before slicing.