In a large pot, heat the coconut oil over medium heat. Add the diced onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic, grated ginger, and lemongrass. Sauté for another 2 minutes until fragrant.
Stir in the red curry paste and cook for 1 minute, allowing the flavors to meld.
Add the chicken broth and coconut milk, stirring to combine. Bring the mixture to a gentle boil.
Add the sliced chicken breasts, red bell pepper, and mushrooms. Reduce the heat to a simmer and cook for 15-20 minutes until the chicken is cooked through and the vegetables are tender.
Stir in the fish sauce and lime juice. Season with salt and pepper to taste.
Remove the lemongrass pieces before serving. Ladle the soup into bowls and garnish with fresh cilantro and lime wedges.
