1. Add water to a Dutch oven to a depth of 1 inch; bring to a simmer. Place chicken in a vegetable steamer. Place steamer in pan over water; cover. Reduce heat to medium-low. Steam, covered, 10 minutes. Remove chicken from steamer; cool. Pat dry.
2. Heat a large skillet over medium-high heat. Add oil; swirl to coat. Add chicken to pan; sauté 5 minutes, turning to brown on all sides. Add cilantro and sliced serrano chiles to pan; sauté 30 seconds, stirring constantly. Remove pan from heat; transfer chicken mixture to a large bowl using a slotted spoon.
3. Bring mango nectar to a boil in a small saucepan over medium heat. Simmer 15 minutes or until reduced to 1/4 cup, stirring occasionally. Stir in lime juice and kosher salt. Drizzle mango mixture over chicken mixture; toss.