Combine 85 grams of hot water with whole wheat flour and your sourdough starter. Stir until evenly combined, then cover and let it sit on the counter until it begins to activate, about 40 minutes.
Meanwhile, combine bread flour, all purpose flour, and 650 grams of hot water in a large bowl. Using your hands or a wooden spoon, stir or knead until no dry flour bits remain, then cover with a dish towel and let it sit on the counter until your starter is ready.
To make the dough, knead the starter into the prepared flour mixture, then gradually add 100 grams of additional hot water and the Kosher salt. Knead gently until all the water is combined, then cover with a dish towel and let it sit on the counter for 45 minutes.
During this time, try to maintain the temperature of the dough at 85°F. If your kitchen is too cold, you can use a larger Dutch oven to create a water bath.
After 45 minutes, perform the first fold, gently pulling the dough toward you and folding it over itself. Repeat for 4 or 5 folds until the dough resists being folded any more. Cover with a dish towel and let it sit for another 45 minutes, again monitoring the temperature.
After 45 more minutes, perform the second fold, repeating your technique from before. Cover again and let it sit for 1 hour.
Using a scraper tool, gently scrape your dough out of the bowl onto the counter—it will still be a little sticky, and carefully stretch it into a large rectangle. (You don’t want to roll it or press out the air you’re already starting to accumulate.) Scatter your chopped peaches and crushed fennel seeds across the surface of the dough, then gently roll up the dough, tucking in any exposed peaches. Return the dough to the large bowl and let it ferment for another hour and a half to two hours, until the dough is bouncy and there are larger bubbles under the surface. Continue to monitor your temperature throughout this ferment.
Scrape the dough onto the counter, divide into two loaves, and carefully shape them. Let them sit on the counter for 1 hour, then transfer to banneton baskets, cover with a towel, and rest for another two hours, until the dough is puffed, then place in the refrigerator overnight.
When you are ready to bake your loaves, preheat your oven to 500°F. Invert your loaf onto the bottom of the bread oven. Using a lame or a razor, carefully score the loaf, then cover and bake for 20 minutes.
Remove the lid and bake for another 18-20 minutes, using tin foil as needed to tent any exposed peaches to keep them from burning.
To make the basil butter, use a food processor to whip the softened butter with the blanched basil, lemon zest, and salt.
