Cut salmon fillet into 1 inch cubes. Slice peeled carrots and celery thinly on the diagonal. Cut onion lengthwise and slice thinly into narrow strips. Remove approx. 2 inches of the woody end of the enoki mushrooms so that the mushrooms are all detached from each other. The vegetables should be a uniform size.
For sauce: Combine all ingredients into a small high-sided bowl. Whisk to combine. Taste the marinade to correct salt/sugar content. Just before starting to cook, pour half the sauce over the salmon cubes.
Preheat vegetable stock to very low simmer in a pot large enough to hold the shanghai noodles.
In a hot wok add a little walnut oil and stir fry the carrots for 3 minutes, adding vegetable stock in small amounts (tablespoon fulls). Add celery and onion with additional stock; stir fry for 4 minutes. Remove vegetables from wok into a warm bowl (at this time, the vegetables are still somewhat crisp). Into the hot wok, pour salmon cubes and their sauce. Allow salmon to cook on high heat for 2 minutes. Return vegetables to the wok and stir fry together. Resist the urge to 'stir' as this will cause the salmon cubes to break apart. Using the high sides of the wok, 'flip' the contents gently until combined (if you are very gentle, you can use a steel wok spatula). Add a little more hot vegetable stock and reduce the heat; simmer for 2 minutes.
Add Shanghai noodles to the simmering vegetable stock. They will separate from each other when they are cooked. Resist the urge to break them apart.
Add sprouts, enoki mushrooms and the remaining sauce to the wok. Simmer for 4-6 minutes using the 'flip' method to combine all items.
Serve the salmon stir fry over the Shanghai noodles; garnish with chopped green onions and toasted sesame seeds.