In a large pot or Dutch oven, heat the vegetable oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Add the curry powder, turmeric, cumin, and red pepper flakes, stirring to combine and toast the spices for about 1 minute.
Add the chicken pieces to the pot, seasoning with salt and pepper. Cook until the chicken is browned on all sides, about 5-7 minutes.
Pour in the coconut milk and chicken broth, then add the fish sauce and brown sugar. Stir well to combine, bringing the mixture to a gentle simmer.
Reduce the heat to low and cover the pot, allowing the curry to simmer for 20-25 minutes, or until the chicken is cooked through and tender.
Stir in the chopped spinach and lime juice, cooking for an additional 2-3 minutes until the spinach is wilted.
Taste and adjust seasoning if necessary. Serve the curry over cooked jasmine rice and garnish with fresh cilantro.