Pat fish fillets dry with paper towels, cut into cubes, and sprinkle all over with kosher salt and black pepper.
To a large pot or Dutch oven over medium high heat, add the oil, diced onion, galangal, lemon grass, cilantro stems, chili peppers, and sauté for 3 minutes.
Add vegetable broth, water, clam juice, fish sauce, chili paste, and sugar. Stir well to combine, and bring to a boil over high heat.
Remove pot from heat and add mushrooms and fish, making sure fish is completely submerged in the broth. Cover with a lid and let sit undisturbed for about 8 to 10 minutes until fish is cooked.
Ladle into bowls and serve with fresh cilantro leaves and a squeeze of fresh lime juice. Serve immediately.