In your large mixing bowl, whisk together the sourdough discard, flour, and salt. Gradually pour in the lukewarm water, mixing with your hands or a spatula until a shaggy dough forms. It will be quite sticky – don't worry, that's perfect!

Drizzle a little olive oil into the bowl and turn the dough to coat it lightly. Cover the bowl with plastic wrap or a damp kitchen towel and let it rise in a warm place for 1-2 hours, or until almost doubled in size.

For extra chewy texture, perform 2-3 sets of stretch and folds during the first hour of rising. Gently grab a portion of the dough, stretch it upwards, and fold it over onto itself. Rotate the bowl and repeat 4-5 times. This strengthens the gluten.

Generously grease your 9x13 inch baking pan with olive oil. Make sure to get into all the corners!

Gently pour the dough into the prepared pan. It will spread out a bit. Cover loosely and let it rise again for another 30-60 minutes, or until puffy and relaxed.

Preheat your oven to 450°F (230°C). Uncover the dough and generously drizzle with more olive oil. Use your fingers to create dimples all over the surface. Sprinkle with chopped rosemary (if using) and flaky sea salt.

Bake in the preheated oven for 20-25 minutes, or until the focaccia is deeply golden brown and sounds hollow when tapped on the bottom.

Let the focaccia cool in the pan for a few minutes before transferring it to a wire rack to cool slightly further. Slice into squares and serve warm or at room temperature.