Prepare the Base:
In a large pot, bring coconut milk and chicken broth to a gentle boil over medium heat.
Add Aromatics:
Add lemongrass, galangal (or ginger), kaffir lime leaves, and Thai chilies. Simmer for 5 minutes to infuse flavors.
Cook the Protein:
Add chicken if using, cook until done (5-7 minutes). For shrimp, add with vegetables for last few minutes of cooking.
Add Vegetables:
Include mushrooms, tomato, and onion (or shallots). Cook for additional 5 minutes until tender and shrimp are pink.
Season the Soup:
Stir in fish sauce (or soy sauce), lime juice, and sugar. Adjust seasoning to taste.
Serve:
Remove lemongrass, galangal, and kaffir lime leaves. Ladle into bowls, garnish with cilantro and green onions.
Enjoy:
Serve hot as is or with steamed jasmine rice on the side.