Let the bread cubes uncovered to dry out completely overnight or dry them in the oven at 350F (180C) for 10-15 min ( make sure they do not brown).
In a large skillet on medium high heat, melt 4 tablespoons of butter and add onions and celery. Cook for about 6 minutes, add garlic and cook for another minute. Season with salt and pepper.
Pour white wine and let it cook until evaporated, about 3 minutes. Add spinach leaves and let them wilt.
In a separate skillet cook the mushrooms in 2 tablespoons of butter until beautifully browned. Season with salt and pepper.
Preheat the oven to 375 F (190C). Brush a 2 1/2-quart (2,5 liters) baking dish with butter.
In a large bowl mix bread cubes with celery onion and spinach mix , add shiitake mushrooms and chopped herbs.
Mix 3 eggs with chicken stock and slowly pour over bread until stuffing feels moist but not soggy.
Transfer the stuffing to the baking dish cover loosely with foil and bake for 20 minutes. Uncover and bake for an additional 20 minutes until browned on top and fully cooked on the inside.