Bring water to a simmer over medium heat. Meanwhile, peel the shrimp reserving the shrimp meat and shells separately. Add salt and shrimp meat to simmering water and cook until shrimp are just pink, about 3 to 5 minutes. Using a slotted spoon, remove shrimp to a plate and refrigerate until ready to use.
Return liquid to stove over low heat, and add shrimp peelings (shells and tails), konbu, shitake, bonito, and ginger. Gently simmer until shrimp shells are pink, konbu has expanded in size, shitake are rehydrated, and bonito are darker in color, about for 5 to 10 minutes. Remove from heat, add soy, and let the broth steep.
Start tasting the broth after it has been steeping for 15 minutes — it should be salty enough that all the flavors are apparent but not so much that you taste the salt. The broth is done when it has a smoky note from the bonito, a slight ginger tone, and a good sea flavor but is not fishy. (I have let this mixture steep as little as 30 minutes and up to 2 1/2 hours.) Strain broth, discard solids, and store until ready to use. (Broth can be made up to 2 days ahead. Store covered in the refrigerator until ready to use.)
For the saimin:
When ready to cook the noodles, bring a large pot of salted water to a boil. Meanwhile, fill a bowl halfway with ice water and set aside. Add broccolini and cook until bright green and knife tender, about 2 minutes. Use a slotted spoon to remove to the ice water bath and set aside. Add noodles and cook according to package instructions until tender. Drain and set aside. (Can be made up to 2 days ahead. Store covered in the refrigerator until ready to use.)
Heat the broth to a simmer over medium heat then divide noodles among four bowls. Top each bowl with top with a quarter each broccolini, spinach, egg, mushroom, Maui onion, green onion, and shrimp. Divide broth evenly among bowls and serve with soy sauce and hot mustard passed on the side.