Heat a large pot over medium heat.
Add coconut milk and bring to a gentle simmer.
Stir in the red curry paste until well blended.
Add the cubed chicken or tofu and cook until browned.
Incorporate the chopped onion and cook until translucent.
Add the sliced bell pepper, green beans, and carrot.
Pour in the fish sauce and brown sugar; stir well.
Simmer for 15 minutes until vegetables are tender.
Adjust seasoning to taste, adding more fish sauce if desired.
Serve hot, garnished with fresh basil leaves.