Heat the vegetable oil in a large pot over medium heat.

Add the onion and cook for 3 to 4 minutes, until softened.

Stir in the garlic, ginger, and red curry paste, then cook for 1 minute until fragrant.

Pour in the chicken broth and coconut milk, stirring to combine.

Add the mushrooms and red bell pepper. Bring the soup to a gentle simmer and cook for 5 to 7 minutes, until the vegetables are tender.

Stir in the shredded chicken, fish sauce, lime juice, brown sugar, salt, and black pepper.

Simmer for 2 to 3 minutes, just until the chicken is heated through.

Remove from the heat and stir in the cilantro. Top with jalapeño if using and serve warm.