Cook dried soba noodles in boiling water with salt. To check for doneness, soba should be al dente (check after 5 minutes). Drain and wash with cold water. Set aside.
In a saucepan, combine soy sauce, sake, dashinomoto, water, sugar and mushrooms. When liquid starts to boil, add meat. Turn off the heat and steep for 20 minutes.
Place soba noodles in individual bowls. Remove meat and mushroom pieces from the hot liquid. Place some meat and mushroom pieces over the soba noodles and pour the hot liquid over. Garnish with sliced leeks on top.