After three minutes, combine with the yeast mixture, all of the flour, semolina, and the rest of the sugar, water, and salt. Whisk until just combined. It should look like a thin batter. Cover with a clean towel and set it aside for 15 minutes. The batter should be full of bubbles after 15 minutes. This batter is very alive, so you want to use it all immediately. The recipe also makes a lot of batter, so if you’re only feeding a few, cut the recipe in half.