Bacon, Cheddar and Carmelized onion Casatiello

Combine the sponge ingredients in a non-reactive mixing bowl, blending thoroughly. Cover and allow to rest 1 hour.
In the bowl of a standing mixer, whisk together the 16 oz bread flour, salt and pepper.
Pour in the sponge mixture and the eggs and mix well on low-medium speed until the dough forms a coarse, shaggy ball.
Continue mixing for 1-2 minutes longer, then turn off the mixer and allow the dough to relax for 15 minutes.
On low speed, begin working the butter into the dough one piece at a time. The dough will be very soft.
Continue beating dough until smooth and velvety, about 12-13 minutes, and the dough comes away from the sides of the bowl. Use the dough hook if you have one and add the cheese, bacon and onions one at a time staggered at 2 minute intervals over the last 6 minutes.
Place in an oiled bowl, turning to grease the top. Cover and allow to rise 90 minutes. The dough will easily triple.
Deflate the dough and shape into a boule. Place in a 9" springform tin that has been lightly greased.
Cover and allow to rise again 45-60 minutes, until it rises over the top of the pan.
Preheat oven to 450F and move the oven rack to the lowest position.
Bake 20 minutes, then rotate pan 180 degrees and bake 25 minutes longer.
Remove from the pan immediately and cool completely on a wire rack.