Start by bringing the broth to a brisk boil in a medium-sized pot over high heat. Once the liquid is bubbling, toss in the lemongrass stalks, kaffir lime leaves, galangal slices, and Thai chilies—each ingredient should be bruised to help release its essential oils and aroma. Lower the heat to medium and allow the mixture to simmer gently for 5 to 7 minutes. This infusion step builds the soup’s signature fragrance and depth.

If you're opting for Tom Yum paste, now’s the time to stir it into the pot. It blends seamlessly into the broth, adding body, color, and an additional layer of rich, spicy complexity.

Drop in the mushrooms and optional onion slices. Let them simmer for 2 to 3 minutes, just until they begin to soften and absorb the broth’s bold flavor.

Gently add the bass fish pieces to the soup, spacing them evenly to avoid overcrowding. Let the fish poach in the hot broth for 3 to 5 minutes. When the flesh turns opaque and flakes apart with ease, it’s ready—be cautious not to overcook, as fish can quickly become dry or rubbery.

Remove the pot from heat and immediately stir in the fish sauce and lime juice. These finishing touches introduce the iconic salty-sour balance of Tom Yum. Taste and adjust—add more lime for tang, fish sauce for umami, or a small pinch of sugar if the citrus is too assertive. Finally, drop in the halved cherry tomatoes, letting them warm briefly in the residual heat without breaking down completely.

To serve, ladle the steaming soup and its vibrant contents into individual bowls. Shower each portion with chopped cilantro, and if desired, a few sliced spring onions and red chilies for extra color and bite. Serve piping hot alongside steamed jasmine rice to round out the meal with comforting simplicity.