Activate starter. 4-6 hours before, mix together 25 grams of starter, 35 grams of all-purpose flour, and 35 grams of filtered water. Stir and let it double and size and become bubbly. This will give you 100 grams of active starter.
Make dough. Add the warm water, active sourdough starter, salt, and sugar in a large mixing bowl. Stir to combine.
Add spelt flour. Add the spelt flour to the water mixture. Stir to combine.
Add all-purpose flour. Next, add the all-purpose flour, using a wooden spoon or spatula, and stir to combine. Form the dough into a ball. Cover and allow to sit for 30 minutes.
Stretch and folds. Wet your hands with water (this helps prevent the dough from sticking to your hands). Pick up one side of the dough, stretch it, and fold the dough over. Turn the bowl 90 degrees and repeat this process until all four corners of the dough have been stretched up and folded over. Cover and allow to sit for 30 minutes. Repeat the stretch and folds 3 or 4 times. Flip the dough over. Cover the dough and allow it to sit for 30 minutes. Repeat the stretch and fold 2-3 more times.
Bulk rise. Cover the dough and allow it to sit at room temperature for 8-12 hours or until it has doubled in size. The temperature of the room will affect the rise time.
Shape. Turn the dough out onto a lightly floured surface. Shape the dough into a ball and place it on a piece of parchment paper. Transfer it to a clean bowl (this will prevent spreading) or sit undisturbed on the counter while your Dutch oven preheats.
Preheat. Place the oven racks in the lower two spots. Place your Dutch oven in the oven and preheat the oven to 450℉. Let the oven sit at 450℉ for 20 minutes to heat your Dutch oven.
Score. Score the top of the dough with a pastry razor.
Bake. Bake at 450℉ covered for 25 minutes. After 25 minutes, remove the lid, reduce the oven temperature to 375℉, and bake for 25 minutes.
Cool. Remove from the oven, transfer to a cooling rack, and cool for at least 20 minutes before cutting into it.