Heat the oil in a medium saucepan over medium-high heat. When the oil is shimmering, add the onion and cook until softened, three minutes. Add the quinoa, salt and pepper and stir to combine. Add the water and bring the mixture to a boil over high heat. Reduce the heat to low, cover and cook until the quinoa is tender and the liquid is absorbed, 15 minutes. Add the mango and toss to combine.