To make the dashi broth, combine the water and the kombu in a large soup pot. Bring to a boil over high heat. Once it has reached a boil, turn off the heat and remove the kombu. You may set the kombu aside for a second stock or dispose of it. Add the bonito flakes and steep for 10 minutes. Strain the broth through a fine sieve.
While you are making your broth, bring a large pot of salted water to a boil for your noodles.
Cook your noodles in boiling water until tender. Drain the noodles and season with a little sesame oil. To reheat the noodles when you are ready to serve, plunge them back into the hot cooking liquid.
To make the salmon glaze: Combine the mirin and sake in small saucepan and simmer for 3 to 4 minutes. Add the ginger, garlic, honey, orange juice and soy sauce; simmer for another 5 to 6 minutes, or until the glaze is syrupy and coats the back of a spoon. Strain the glaze and set aside to slightly cool. Once the glaze has cooled, brush the salmon fillets with the glaze using a pastry brush. Either grill or broil the salmon for 2 minutes. Remove from the heat and brush again with the glaze. Repeat this process 2 or 3 more times, until the salmon is cooked to the desired degree of doneness and nicely glazed.
To make the kakejiru broth, in a medium sauce pot, combine the dashi broth with the soy, sugar, and salt and bring to a boil.
Place the hot soba noodles in a bowl, pour the hot kakejiru broth over the noodles, and place the salmon on top of the noodles. Garnish the noodles with the togarishi, toasted nori, sesame seeds and scallions.