Soak the raisins in 2 cups of hot water for 1-2 hours. Meanwhile, in a large mixing bowl combine the flour and water, cover with plastic wrap and let it sit for 30 minutes. Then add the starter and salt, mix to combine, cover with a plastic wrap and let it sit for 30 minutes longer.
Then uncover the dough and perform the first set of stretch and folds by picking up the dough with your hand, stretching it as far as it will go without tearing and folding onto itself, then rotate the bowl 45 degrees, pick up another bit of the dough, stretch and fold again, then rotate 45 degrees again and repeat, rotate and repeat for the last time. You should stretch and fold your dough 4 times in total per set.
Perform 3 sets of stretch in fold 30 minutes apart. Before you start the last set of stretch and fold add in your soaked and drained raisins and chopped walnuts and work them into the dough during the stretch and fold. Be gentle. Cover the dough with plastic wrap and let it ferment for 5-7 hours. (The fermentation process might happen quicker or slower depending on how warm or cold your house is.) By the end of of it, the dough should be slightly rounded on top and form a few bubbles on the surface. It will NOT double in size.
Carefully transfer the dough onto a floured surface and shape into a ball by tucking the sides inward. You want to create tension on the surface of the dough. Then carefully transfer the shaped boule into the proofing basket seam side up. Make sure your proofing basket is generously dusted with rice flour, this will ensure the dough doesn't stick to it. Cover with a tea towel and refrigerate for 14-16 hours. I usually do it overnight. (Watch the shaping technique here)
The following day preheat the oven with the Dutch oven inside to 450F/220C for one hour. Turn the dough out of the banneton to a parchment paper lined sheet, then score with a sharp blade. Transfer the bread to the preheated Dutch Oven by carefully picking it by the corners of parchment paper, cover with a lid and bake for 30 minutes. Then take the lid off and bake for 15 minutes longer.
Cool the sourdough bread on a wire rack for at least 4 hours before slicing.