In large bowl, whisk together bread flour, all-purpose flour, salt, yeast and sugar. Drizzle in water and 3 tbsp of the olive oil, mixing with a wooden spoon until a shaggy dough forms and no dry streaks remain. The dough will be very wet and sticky; this will yield a fluffy focaccia. Cover bowl with a kitchen towel and let stand 5 minutes.

Using wet hands, perform a set of stretch and folds. Gather up edge of dough, lift it and fold it over on itself and then rotate bowl 90 degrees; repeat three more times.

Spread butter on bottom of 13 x 9-in baking pan to prevent sticking, then drizzle in remaining 3 tbsp olive oil. Transfer dough to prepared pan, spreading slightly.

Let rise, uncovered, in a warm area until doubled in size and jiggly, 1½ to 3 hours (depending on room temperature).

Preheat oven to 450°F 30 minutes before the end of the second rise. Drizzle remaining 2 tbsp olive oil over dough; using oiled fingertips, dimple the dough all over, pressing down to touch bottom of pan. Press in rosemary leaves or choice of topping and sprinkle with flaky sea salt (if using).

Bake until golden brown and edges pull away from pan, 25 to 30 minutes. Let cool in pan for 5 minutes before transferring to rack.

Let cool slightly before slicing with a bread knife.