In a large pot, heat the olive oil over medium heat. Once hot, add the diced chicken thighs and season with salt and pepper. Cook for approximately 5-7 minutes, stirring occasionally, until the chicken is nicely browned all over. Remove the chicken from the pot and set aside on a plate.
In the same pot, reduce the heat slightly and add the finely chopped onion, minced garlic, and grated ginger. Sauté for 3-4 minutes, stirring frequently, until the mixture is fragrant and the onion has become translucent.
Stir in the diced red bell pepper and allow it to cook with the aromatics for an additional 2 minutes, stirring occasionally to prevent sticking.
Pour the can of coconut milk and the chicken broth into the pot, stirring well to combine all ingredients. Increase the heat to bring the mixture to a gentle simmer.
Return the cooked chicken to the pot, then add the fish sauce, fresh lime juice, and brown sugar. Stir well again and let the soup simmer for about 10 minutes to allow the flavors to meld.
Just before serving, add the baby spinach to the pot, stirring it through the hot soup until it wilts, which should take about 1-2 minutes.
Taste the soup and adjust the seasoning, adding more salt, pepper, or lime juice if required, to suit your palate.
Ladle the piping hot soup into bowls. Garnish each bowl with fresh cilantro on top and serve with lime wedges on the side for an added zesty touch.
Notes
For an elegant touch, serve the soup in shallow bowls and scatter a few cilantro leaves on top just before serving.