Place water and stock powder in a large saucepan over high heat. Bring to boil. Add miso paste and stir until the miso dissolves. Reduce heat to medium-low and hold at a gentle simmer.
Add the noodles and cook for 1-2 minutes or until just soft. Add the shallot and choy sum and cook for 1 minute or until the choy sum just wilts.
Meanwhile, heat a non-stick frying pan over medium-high heat. Spray with oil. Cook the salmon for 2-3 minutes each side for medium or until the fish flakes easily when tested with a fork. Sprinkle with the sesame seeds.
Flake the salmon into large pieces. Divide the broth and noodles among serving bowls. Drizzle with soy sauce and top with extra shallot.