To prepare chicken, combine first 10 ingredients in a food processor; pulse 30 times or until finely chopped. Spoon onion mixture into a large zip-top plastic bag. Add chicken to bag; seal. Marinate in refrigerator overnight, turning bag occasionally.
Remove chicken from bag; discard marinade. Sprinkle chicken with 1/2 teaspoon salt. Place chicken on a grill rack coated with cooking spray; grill 6 minutes on each side or until done.
To prepare plantains, peel plantains, and cut into 1-inch pieces. Cook plantains in boiling water 20 minutes or until tender; drain. Combine plantains, coconut milk, butter, and 1/4 teaspoon salt in a medium bowl; mash to desired consistency. Place 1/3 cup plantain mixture on each of 6 plates, and top each serving with 1 chicken breast half.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.