In the bowl of a stand mixer fitted with a dough hook, combine the warm water and sugar, then sprinkle with the yeast. Let sit 5 minutes, until foamy. If the yeast does not foam, your yeast is dead and you will need to start over with fresh yeast.
Add the all-purpose flour, sourdough discard, chopped rosemary, salt and pepper. Mix until combined and a shaggy dough forms. Transfer the dough to a large, oiled bowl, then turn to the dough to coat with the oil. Cover with plastic wrap and let rise 2-3 hours, until doubled in size.
Brush a 13x9 inch pan generously with 3 Tablespoons olive oil. Set aside.
Once the dough has completed the first rise, use a spatula to scrape around the edge of the bowl, deflating the dough. Transfer the dough to center of the prepared baking pan. Cover with a clean kitchen towel and let rise 1 ½-2 hours. At the end of this rise, the dough should completely fill (or nearly fill) the baking pan.
Preheat the oven to 450°F. Drizzle the dough with 2 Tablespoons olive oil, then using your fingers, press dimples into the dough all over. Sprinkle with flaky sea salt.
Bake 25-30 minutes until the focaccia is puffed, golden and crusty. Remove from the oven and drizzle with an additional 1-2 Tablespoons olive oil. Let rest in the pan for 10 minutes, then transfer to a cooling rack to cool fully. Slice and enjoy!
Video
Notes
To make this recipe with active sourdough starter, omit the yeast and replace the sourdough discard with active starter (200g). You will also need to increase the rise times.