In a large bowl, mix 3 cups of flour (a mix of all-purpose and whole wheat) with 1/2 cup of active sourdough starter, 2 tablespoons of olive oil, and 1 cup of lukewarm water until it forms a shaggy dough.
Let the dough rest covered for 30 minutes at room temperature.
Fold in 2 teaspoons of salt, 1 cup of chopped walnuts, and 1 cup of dried cranberries into the dough.
Cover the bowl and let the dough rise at room temperature. Perform stretch and folds every 30 minutes for the first 2 hours.
Continue to let the dough rise until it has doubled in size, which may take 4 to 8 hours depending on the room temperature.
Turn the dough onto a lightly floured surface and shape it into a round loaf or into a rectangle if using a loaf pan.
Place the shaped dough into a floured banneton basket or a loaf pan lined with parchment paper for the final rise.
Cover and let the dough rise for 1 to 2 hours at room temperature, until it looks puffy and has increased in volume.
Preheat the oven and a Dutch oven to 450°F (232°C). Transfer the dough into the hot Dutch oven, score the top, and bake with the lid on for 25 minutes.
Remove the lid, reduce the oven temperature to 425°F (218°C), and bake for another 20-25 minutes until golden brown.
Cool the bread on a cooling rack for at least an hour before slicing.