Heat a large pan over medium heat and add a splash of oil.
Add the chopped onion and sauté until translucent.
Stir in garlic and ginger, cooking for an additional minute.
Add the chicken thighs and brown them on both sides.
Mix in the red curry paste, ensuring the chicken is well coated.
Pour in the coconut milk and bring the mixture to a simmer.
Add the sliced bell pepper and green beans to the pan.
Stir in fish sauce and let the curry simmer for 20 minutes.
Check the chicken for doneness; it should reach an internal temperature of 165°F (74°C).
Garnish with fresh cilantro before serving.