Heat the butter over medium heat in a dutch oven or large saucepan.
Add the clams, cover and steam for about 2 minutes. Add the garlic and shrimp, cover and steam another 2 minutes, or until all the clams have opened and the shrimp has started to turn pink.
Remove shrimp and clams with a slotted spoon and set aside. Discard any clams that have not opened.
Add the water, dashi powder, sake, mirin and miso. Simmer for 5 minutes.
While the broth is simmering, heat the udon noodles in boiling water for 1 minute. Divide the warm noodles into 4 bowls.
Add the enoki mushroom, shrimp, and clams to the bowls.
Add the fish to the broth and cook for 30 seconds to 1 minutes. Remove from broth and divide among soup bowls.
Ladel the broth into each bowl. Top with sliced fish cake, poached egg (optional) and sliced green onions.