In a large pot, heat a tablespoon of oil over medium heat. Add the diced chicken thighs and cook until browned on all sides, about 5-7 minutes. Remove and set aside.
In the same pot, add the chopped onion, garlic, and grated ginger. Sauté for 3-4 minutes until the onion is translucent and fragrant.
Pour in the coconut milk and chicken broth, stirring well to combine. Add the lime juice and zest, fish sauce, brown sugar, and red pepper flakes. Bring the mixture to a gentle simmer.
Return the browned chicken to the pot and allow it to cook in the broth for about 10 minutes, uncovered.
Add the diced red bell pepper and snap peas to the soup, cooking for another 5 minutes until the vegetables are tender yet crisp.
Taste and season with salt and pepper as necessary. Adjust lime juice and red pepper flakes for desired flavor.
Serve the soup hot, garnished with fresh cilantro.
Notes
Adjust the spice level with red pepper flakes to taste.