Whisk rye sourdough starter with warm water until frothy. Add molasses and whisk until it's dissolved.
Add sifted whole grain rye and spelt flour, cardamom, cinnamon and salt; mix with a spoon until no dry bits remain. Add raisins and walnuts, using a spoon or a wet hand (easier).
Cover tightly, and leave at room temperature for 6-8 hours, or overnight.
Using wet hands, shape the dough into a ball using a motion as if you are tucking the dough underneath itself, and place it on parchment paper. Spray with water, and cover with plastic wrap. Leave for an hour or two. The dough will spread out and puff up.
When ready to bake, place a pizza stone into the oven and preheat it on to 425ºF. When the oven is hot, place the dough with parchment paper on the pizza stone (I slide it off a cutting board), generously spray the dough and pizza stone with water. Bake for 20 minutes.
Turn the temperature down to 375ºF and bake 15-20 more minutes. Spray the dough with water a couple of times during baking, it will give the crust a nice sheen.