Put the flours and the yeast in a large mixing bowl and stir together. Add the water gradually, and stir with a wooden spoon until the mixture gathers into a rough, sticky dough. Sprinkle the dough lightly with a little more flour, and knead the dough in the bowl for a 2–3 minutes. Round the dough into a ball and before you put it back in the bowl, oil the bowl lightly to prevent the dough from sticking to the bow.
Cover with plastic wrap or a damp towel, and put in a warm place to rise for 2 hours. (You can refrigerate the dough overnight.)
Line a large baking pan (10" x 15") with a baking sheet and drizzle it with olive oil. Remove the dough from the mixing bowl (even if it's still cold from fridge) and pat and press it into the baking pan. It might spring back a bit – if it does, just wait a few minutes – eventually it will relax and become more soft and stretchy.
Cover with a damp towel or plastic wrap and let it rise again in a warm place for about an hour.
Preheat the oven to 400°F. Scatter the scallions and rosemary evenly across the top of the dough and drizzle with olive oil. Poke little dimples evenly over the top of the focaccia, and sprinkle with coarse salt.
Bake for 25–30 minutes, until it's nicely browned on top and the bottom seems done. Cool to room temperature, "if you can wait," as the admirable cook writes in his book.