The next day, drizzle an 9x13 baking pan with olive oil, and spread the oil to evenly coat the bottom and sides of the pan.
Remove the dough from the fridge, and loosen it from the sides of bowl (it may be sticky). Transfer the dough, upside down, into the baking pan.
Pat the dough into a rough rectangle shape inside of the pan, gently stretching the top and bottom if needed. Now, we're going to shape the dough.
Using both hands, gently pick up the bottom of the dough, pull it away from the pan, and fold it halfway on top of the dough. Next, pull the right side of the dough away from the pan, and fold it halfway on top of the dough again, overlapping the bottom portion. Repeat with the left side of the dough, and then finally the top, so you have a nice little square of dough.
Place your thumbs just underneath the bottom of the dough, and gently roll the dough over on top of itself, liking rolling a towel, so that it's flipped over. The surface of the dough should now be nice and smooth, with maybe a few air bubbles. Gently turn the dough sideways, if needed, so it's resting neatly in the center of the pan.
Loosely cover the pan with a towel, and rest at room temperature for about 4 hours, so that the dough naturally spreads to fit the pan.
Preheat the oven to 450 F.
Drizzle olive oil in top of the dough, and lightly oil your fingers. Dimple the dough all over with your fingers, poking through to the bottom of the pan, until the surface is covered in holes and there's lots of air bubbles.
Sprinkle the top of the dough with rosemary and pinches of sea salt.
Bake the sourdough focaccia for 25-30 minutes, until the top is golden brown.
Meanwhile, prepare the Garlic Butter by melting the butter in a small pan over medium heat. Add the minced parsley and garlic. Cook for about 1-2 minutes. You just want to wilt the parsley and soften the garlic a bit, but be careful not to burn it. Remove the pan from heat and allow it to cool slightly. Transfer the garlic butter to a bowl and set aside.
Remove the sourdough focaccia from the oven. Brush the hot sourdough focaccia with the garlic butter. Cool for at 30 minutes before slicing.