Make the chicken: Heat the vegetable oil in a large skillet over medium-high heat. Add all but 1/2 cup onion, season with 1/2 teaspoon each salt and pepper and cook, stirring, until translucent, 5 minutes. Stir in the minced serrano, chile powder and zest and juice of 1 lime; stir 30 seconds. Stir in the chicken broth and chicken and bring to a simmer. Reduce the heat to medium low and cook until thickened, about 8 minutes.
Meanwhile, make the mango salsa: Combine the mango, sliced serrano, reserved 1/2 cup diced onion, the tomato, cilantro, the juice of the remaining lime and 1/2 teaspoon salt in a medium bowl.
Make the cheese sauce: Melt the butter in a medium saucepan over medium-high heat. Whisk in the flour and cook until foaming, about 2 minutes. Whisk in the milk and bring to a simmer, then reduce the heat to medium low and cook, stirring, until thickened, about 4 minutes. Remove from the heat and stir in the cheese until smooth.
Reheat the chicken mixture, if necessary. Arrange the plantain chips on a platter. Top with half of the cheese sauce, the chicken, remaining cheese sauce, guacamole and mango salsa.