1.Soak mushrooms in boiling water 20 minutes; drain. Discard stems; slice caps thinly.
2.Meanwhile, soak seaweed in boiling water 5 minutes, drain.
3.Shell and devein prawns; chop coarsely.
4.Cook noodles in large saucepan of boiling water until tender; drain.
5.Meanwhile, combine dashi and the boiling water in large saucepan; bring to the boil. Place paste in small bowl; whisk in 1 cup of hot dashi stock. Add paste mixture to dashi stock in pan; whisk until combined. Bring to the boil; add prawns and snapper and simmer until seafood is cooked through.
6.Divide noodles among six serving bowls, add mushrooms, seaweed, tofu and onion; ladle over miso broth.