Pick over buckwheat and remove any small stones. Rinse and drain. Add mushroom broth and buckwheat to a medium saucepan and cook on medium until most of the water is absorbed. Turn off heat, cover and let sit covered until all the water is absorbed, about 20 minutes.
While buckwheat is cooking, heat olive oil in a medium saucepan. Add onion, celery and garlic. Cook a few minutes until vegetables are soft and getting some colour. Add spices, chopped chestnuts, cooked buckwheat and dried cranberries. Taste for seasoning and add more salt and pepper if needed.
Heat 1 tsp oil in small pan and quickly fry sage leaves. Sprinkle on top.
Preheat oven to 350 degrees. Cut bread into small cubes and add into mixture. Pour extra broth in and mix well. Press into oven proof casserole dish or pan. Bake for 30 minutes.