Weigh the water, stout, onion juice, salt, oil, poolish, bread yeast into a container. Blend the ingredients together with a whisk to incorporate them well and dissolve the salt. Add this to the mixer bowl, then top with the flour ingredients. Using a dough hook, turn the mixer on low speed and set a timer for 10 minutes. Let the dough come together, while you prepare the next batch of dough.