In the bowl of a stand mixer stir together water, sugar and yeast. Allow to sit for 10 minutes until foamy. To the yeast mixture add sautéed onion, 1 egg, butter, salt and 2 cups of bread flour. Stir to thoroughly combine. Set mixing bowl on stand mixer fitted with dough hook and knead mixture at low speed adding flour a 1/4 cup at a time. Allow all flour to be absorbed before adding more. Knead until dough pulls away from the sides and bottom of bowl, approximately 8-10 minutes.