Heat the vegetable oil in a large pan over medium heat.
Add the green curry paste and sauté for 1-2 minutes until fragrant.
Pour in the coconut milk and stir well to combine.
Add the sliced chicken (or tofu) and cook until almost done, about 5 minutes.
Toss in the bell pepper and green beans, cooking for another 5 minutes.
Stir in the spinach and let it wilt.
Add fish sauce and brown sugar, adjusting to taste.
Simmer the curry for an additional 5 minutes to develop flavors.
Remove from heat and garnish with fresh basil leaves.
Serve hot with steamed jasmine rice.