Rinse cod, and soak and discard water, 4-6 hours each time, for a total of four soaks, to remove all salt.
In saucepan, put cod, 1/2 cup green onions, shrimp paste, chili paste, fish sauce, sugar, ginger powder, garlic powder, 2 tablespoons lime juice, white pepper, lemon grass paste, water (everything in the ingredients up to and including water).
Bring to a simmer and cook for 10 minutes.
Place dried mushrooms in bowl, pour boiling water over them, and let sit for 10 minutes to rehydrate. Do not drain.
Add salt, rehydrated mushrooms (including their soaking water), and tomatoes to soup. Let simmer another 10 minutes, and then turn off the heat.
Add 1 tablespoon lime juice, remaining 1/2 cup of green onions. Stir in evaporated milk.
Serve over rice for a filling meal, or as a soup alone for an appetizer or something lighter.