1. In a large pot over medium heat, pour the vegetable oil. Add in the finely chopped onion, minced garlic, and sliced red chili. Sauté for 2-3 minutes until the onion is soft and fragrant.

2. Stir in the red Thai curry paste and cook for 1 minute, stirring continuously, until aromatic.

3. Pour in the chicken broth and coconut milk. Stir to combine well.

4. Add the whole chicken breasts and bring to a gentle simmer. Cover and cook for 15 minutes until the chicken is cooked through.

5. Remove the chicken and shred it using two forks.

6. Return the shredded chicken to the pot. Add the diced red bell pepper and sliced mushrooms. Simmer for another 5 minutes until the vegetables are tender.

7. Stir in the fish sauce, brown sugar, and lime juice. Taste and adjust seasonings as needed.

8. Ladle the soup into bowls and garnish with chopped cilantro and optional basil.

9. Serve hot with lime wedges on the side.