Soak mushrooms in a medium bowl of hot water, covered, until soft, about 1 hour. Drain, trim off and discard stems, thinly slice caps, and set aside. Meanwhile, soak noodles in a medium bowl of warm water until pliable, about 15 minutes. Drain and set aside. Cut white parts of scallions lengthwise into thin strips and set aside; set remaining scallions aside separately.
Rub fish inside and out with a few generous pinches of salt, then rinse off under cold running water. Using a sharp knife, score fish in 3 places, about 1" apart, on both sides. Reserve a few strips of the white parts of the scallions and tuck the remaining white parts into incisions. Put fish on a heatproof plate large enough to hold fish and small enough to fit inside a large steamer basket, and set aside.
Heat oil in a large skillet over medium-high heat. Add onions and ginger and cook, stirring, for 1 minute. Add pork and mushrooms and cook, breaking up meat with a wooden spoon, until no longer pink, about 2 minutes. Turn off heat, stir in nu'ó'c mắm, noodles, and reserved scallion greens, and season to taste with pepper. Tuck half of the meat mixture inside fish cavity, spoon the remaining half over fish, and put tomatoes on top.
Set steamer basket with fish over steamer pot of boiling water over high heat, cover and steam until fish is just cooked through, about 15 minutes. Garnish with cilantro and reserved scallions.