To make soup broth: In a medium stockpot, combine the dashi, sake, mirin and soy sauce. Bring to a boil. Remove from the heat, cover and set aside.
Heat the oil in a stockpot over medium heat. Add the sliced mushrooms and sauté until soft. Add the garlic, ginger and green onions; cook for 1 minute or until fragrant. Stir in the soup broth and bring to a boil. Remove from the heat, cover and set aside.
To cook soba noodles: In a pot of boiling water, cook the soba noodles according to package instruction. Rinse under cold running water and then drain.
To serve: Divide the noodles among four individual bowls. Pour hot soup over the soba noodles. Top with roasted seaweed flakes and green onion. Sprinkle with shichimi togarachi, if desired.