In a large bowl, whisk together lukewarm water, sourdough discard, honey, and instant yeast. Add the bread flour and fine sea salt. Mix with a spatula until a shaggy, sticky dough forms. Pour two tablespoons of olive oil around the sides of the bowl and turn the dough to coat.

Cover the bowl and let rise at room temperature for 8-12 hours, or overnight, until doubled in size and bubbly.

Pour a generous amount of olive oil into a 9x13 inch baking pan. Transfer the dough to the pan and gently stretch it out. Let it rest for 30 minutes if it resists. Cover and let it rise for another 1-2 hours until puffy.

Preheat oven to 425°F (220°C). Drizzle the dough with more olive oil. Oil your fingertips and press down firmly to create dimples all over the surface.

Sprinkle generously with flaky sea salt and fresh rosemary (if using). Bake for 20-25 minutes until the top is deep golden brown.

Let the focaccia cool in the pan for a few minutes before transferring to a wire rack. Slice and serve warm.