Mix bread flour, whole wheat flour, water, brine, and starter in a bowl until no dry flour remains. Cover and rest 30 minutes. Sprinkle salt, dill, and pickles over the dough and fold until evenly distributed.

Cover and rest 30 minutes. Perform a stretch-and-fold series. Repeat 3 to 4 more times at 30-minute intervals, then let the dough bulk ferment until increased by 50% and jiggly.

Turn the dough onto a floured surface, pre-shape into a round, and rest 20 minutes. Shape into a tight boule and place seam side up in a floured banneton.

Cover and proof overnight in the refrigerator for 12 to 16 hours, or at room temperature for 1 to 2 hours until puffy. Preheat oven with a Dutch oven inside to 450°F (230°C). Turn loaf onto parchment, score deeply.

Lower dough into the hot Dutch oven. Bake with lid on for 25 minutes, then remove lid and bake 20 minutes more until deep golden and hollow-sounding. Cool fully on a rack, then slice and serve.