In a blender, combine 2 cups of the frozen mangoes, onion, garlic cloves, brown sugar, rice vinegar, ginger, red pepper flakes, curry paste and chicken broth. Blend until smooth and transfer to the slow cooker.
Stir in coconut milk and Sazon in curry until combined.
Add the chicken and red onion and gently toss so everything is coated with the curry sauce.
Cook on low for 8 hours or high for 4 hours.
Add the remaining mangoes and cover to thaw, approximately 10 minutes.
Add the cashews and serve over rice, garnished with fresh cilantro, red chili and/or lime wedges.