Mix the starter ingredients in a jar, cover loosely, and leave to peak (4 to 6 hours).
Toast the walnuts in a dry pan over medium heat for 5 to 7 minutes until fragrant. Cool and break into rough pieces. Chop the figs. If figs are hard, soak in warm water for 10 to 15 minutes, drain, and pat dry before chopping.
Combine the flours and water into a shaggy dough. Cover and autolyse for around 30 minutes.
Add the leaven and squish in with wet hands. Add the salt and mix in.
Over the next 3 hours, do a set of stretch and folds every 30 minutes. After about four sets, add half the figs and walnuts, fold in, then add the rest and fold in.
Leave the dough to finish bulk in a warm spot (ideally 25°C / 77°F) until bubbly, puffed, and jiggly, another 1 to 2 hours depending on room temperature.
Turn out the dough gently, pat into a rectangle. Fold the bottom third up to the middle, fold the right side in, then the left side in, then bring the top third down over everything to make a parcel. Flip seam-side down and gently roll towards you on the bench to build surface tension.
Place seam-side up in a floured banneton. Cover and cold proof in the fridge for 8 to 16 hours.
Preheat the oven and a Dutch oven to 230°C / 450°F for at least 30 minutes. Flip the dough into the hot Dutch oven (or onto baking paper to lower in). Score.
Bake with the lid on for 20 minutes, then remove the lid and bake for another 20 minutes until deeply browned. Cool on a wire rack for at least 2 hours before slicing.