Salmon head soup with lemongrass, ginger and lime juice

Name: Salmon head soup with lemongrass, ginger and lime juice
Prep time: 10 min
Cook time: 20 min
Total time: 30 min
Ingredients: 3 tablespoons cooking oil; 1 2-inch knob ginger (peeled and thinly sliced); 4 cloves garlic (peeled and minced); ¼ cup thinly sliced scallions ((white and light green portions only) ); 3 to 4 stalks lemongrass ((white portion only) peeled and thinly sliced); 4 tomatoes (diced); fish sauce; ground black pepper; 1 kilogram salmon head (chopped through the bone into eight pieces (see notes)); 2 eggplants (diced); 2 to 4 tablespoons lime juice ((lemon or calamansi juice may be substituted)); ⅓ cup thinly sliced scallions ((dark green portion only)); cilantro (to garnish)