Fry the trout in plenty of oil in a wok until skin is crispy, about 2-3 minutes on each side. Set aside.
In a soup pot heat the soup stock to a simmer. Add galangal, lemon grass and keffir lime leaves to infuse the stock for about 10 minutes.
Take out the herbs. Add tomatoes and season soup with fish sauce and lime juice. Simmer for 5 minutes or until tomatoes start to soften.
Add fish (you might have to cut it into 2 or 3 sections to fit the pot) and roasted chili paste. Bring soup back to a quick boil. Garnish with coriander leaves. Serve immediately as part of a family meal with lots of rice.