Heat 1 tablespoon vegetable oil in a large saucepan over medium heat. Add onion; cook and stir until the onion softens and turns translucent, about 5 minutes. Add mangos, lemon juice, sugar, ginger, cinnamon, and mace. Bring to a simmer and cook for 5 minutes, stirring constantly. Turn heat to low, cover, and allow to cook 5 minutes more.
While the mango sauce is simmering, season chicken breasts with salt and pepper to taste. Heat remaining 1 tablespoon vegetable oil in a large skillet over medium heat. Add chicken, and cook on both sides until nicely browned, and the chicken is no longer pink in the center, about 8 minutes per side.
To serve, place one or two pieces of chicken onto each serving plate. Spoon mango sauce over the top, and sprinkle with pecans.
