Place 1¼ cups lukewarm water in a small bowl. Sprinkle with 2 packets active dry or instant yeast and 1/2 tsp. of the granulated sugar, and stir to combine. Let sit until foamy on top, about 5 minutes. If it doesn’t smelly “yeasty” or bubble up, throw it out and start again with new yeast. Meanwhile, place 1½ cups of the bread flour, the remaining ¾ cup granulated sugar, and 2½ tsp. kosher salt in the bowl of a stand mixer.
Brush the egg wash liberally over the challah. Sprinkle each loaf with 1 to 2 Tbsp. of chopped rosemary, and a 1 Tbsp of Turbinado sugar, if desired.