Soak wakame seaweed in water to cover, and let sit 10 minutes to rehydrate.
Bring dashi to a boil in a medium size pan. Mix miso with some dashi to make a thin paste.
Stir miso into dashi, and add rehydrated wakame seaweed, mushrooms and tofu. Bring to a boil, then remove from heat.
Serve miso soup immediately, garnished with scallions.