Prepare a spice sachet by bundling lemongrass, galangal (or ginger), kaffir lime leaves (or lime zest), and crushed bird eye red chilies in cheesecloth and tying with string.

Heat coconut milk and water in a saucepan and bring to a boil.

Add the spice sachet and reduce heat to a simmer.

Add sliced chicken, chopped oyster mushrooms, and thinly sliced onion to the simmering liquid.

Simmer for approximately 5 minutes, then add roughly chopped tomato.

Ensure the chicken is fully cooked.

Taste and season with fish sauce, sugar, and fresh lime juice, adjusting to personal preference.

Cook until all ingredients are heated through but the vegetables remain intact.

Serve immediately, garnishing with Thai basil chiffonade or fresh, chopped cilantro.