Add 480g bread flour, 120g whole-grain spelt flour, 14g salt to a bowl.
Mix the ingredients with your hand until they are all incorporated.
Add 468g water and 120g mature sourdough starter to the bowl.
Mix the dough until there is no dry flour left.
Let the dough rest for 1 hour to let the gluten network start forming.
Perform a set of stretch and folds. Let the dough rest for 30 minutes.
Perform a set of stretch and folds. Before each fold, add ¼ of the cheddar, jalapeños, and chives. Let the dough rest for 30 minutes.
Perform a set of stretch and folds, move the dough to a see-through bulking container, where you can monitor the growth and let the dough grow 25%.
Divide and preshape
Divide the dough into two equally sized pieces.
Preshape both pieces of dough into balls. Let them rest on the counter for 20 minutes.
Shape the two balls into batards and put them in oval rice flour-dusted proofing baskets.
Put the proofing baskets in each bag and put them in the fridge to retard.
Let me retard for at least 8 hours, up to 48 hours.
Preheat your oven to 230°C/450°F, including your favorite baking vessel (cast iron pot, pyrex dish, or whatever) and a baking steel/stone. You can use a fan or no fan; it doesn't make a difference. After your oven hits the temperature, you should continue to heat for at least 30 minutes.
Take out one of the doughs and sprinkle rice flour on the bottom of the dough.
Flip the dough out onto your peel.
Using a brush or your hands, remove any rice flour left on the top of the dough.
Score the dough with whatever pattern you'd like.
Add the dough to the oven and put the baking vessel over the top.
Bake for 20 minutes.
Open the oven and remove the vessel.
Bake until the bread is done. This takes 20 minutes in my oven but may take longer or shorter in yours.
Bake the other bread the same way and let them cool off completely before using.