In the bowl of a stand mixer, combine sourdough starter, water, flour and salt. Beat with a dough hook at medium speed for 3 minutes until a rough, shaggy ball is formed. Pull dough off hook and place in the plastic container.
Cover and leave on counter for one hour. After an hour has passed, pull and fold over dough and cover again. Repeat this two times (over 2 hours). After folding over the second time, put container in the fridge or in a cold garage and leave to sit overnight or at least 8 hours.
After 8+ hours of resting, bring container back to room temperature and allow the dough to rise and rest for another 3 hours at room temperature.
Start grill or oven to 400° and place lidded dutch oven in grill to preheat as well. Allow to preheat for at least 45 minutes-- the pan should be screaming hot.
After oven and pan have been preheating for 30 minutes, prepare dough for baking. Remove from plastic container and dust liberally with additional flour. Gently knead/form into a ball and place on the center of parchment square.
Using razor blade, cut several deep cuts into surface of dough, in whatever pattern/design you would like. The cuts in the dough help with steam release but also provides a nice look at the end.
Carefully remove lid from preheated dutch oven and drop dough (on parchment) into the center of the pot. Replace lid and put back in oven/grill for 45 minutes.
After 45 minutes, remove lid and allow to cook for 15 more minutes, until browned.
Allow to cool slightly before slicing, and be sure to enjoy a warm slice with softened butter!