Preheat oven to 350 degrees.
Combine all ingredients except the brown sugar and ¼ cup walnuts. Stir to combine. The combination of baking soda and sourdough starter will cause the dough to bubble up.
Grease a 10-12 cup Bundt pan.Sprinkle the bottom with the brown sugar and ¼ cup walnuts.
Spoon the batter into the pan and bake for 40-50 minutes or until a toothpick inserted in the middle comes out clean.
Remove from oven. Cool on a rack for10 minutes; then turn out of the pan and cool completely before slicing.