Pour the chicken stock or water into a medium-sized pot and bring to a rolling boil over medium heat. Add the lemongrass, galangal slices, torn kaffir lime leaves, and red chili peppers to the pot. Allow the aromatics to simmer for 3–4 minutes, releasing their bold, fragrant oils into the broth.

Gently place the prawns, mussels, and crab claws (if using) into the pot. Let the seafood simmer for 5–7 minutes, ensuring the prawns turn pink and opaque, and the mussels open fully. Discard any mussels that remain closed, as they are not safe to eat.

Add the diced tomato, red bell pepper, and halved mushrooms to the broth. Cook these vegetables for 3–4 minutes, just until they soften slightly while retaining a bit of their texture for a delightful contrast in the soup.

Stir in the Thai chili paste, fish sauce, lime juice, and sugar. Mix well to dissolve the chili paste completely, and adjust the seasoning to achieve the perfect harmony of spicy, sour, salty, and sweet flavors. Simmer the soup for another 2–3 minutes, allowing the flavors to meld together.

Serve hot, garnished with fresh coriander leaves.