To make the dough:
Whisk the starter, water, and honey in a large bowl. Add the flour and salt. Mix with a fork and then continue to combine with your hands until the flour is absorbed.
Bulk Rise:
Cover the bowl with plastic wrap and let rise overnight, at room temperature, about 12-18 hours. The dough is ready when it no longer looks dense and has doubled in size and look stringy. There may also be bubbles on the surface as well.
Second Rise:
In your 9×13 pan pour 2 tablespoons of olive oil and spread around to coat the bottom. Dump the dough onto the pan. Turn over to coat both sides with oil. No need to stretch or spread the dough out yet. Cover the pan with plastic wrap and let rest for 1 1/2 to 2 hours until very puffy. You can let it rise overnight again which I did but not necessary.
Assemble the Focaccia:
Gently stretch the dough to reach the corners of the 9×13 pan. Use your fingertips to dimple the dough. Drizzle the remaining 2 tablespoons of olive oil and sprinkle on 2 Tablespoons of chopped fresh rosemary. Gently press the rosemary leaves into the dough. Sprinkle with additional flaky sea salt (optional).
Bake:
Bake the focaccia on the center rack, in a preheated 425 degrees oven, for 20-25 minutes until crispy and golden brown. Remove from the oven and cool before cutting into wedges using a serrated knife. Serve warm.