Heat the oil in a large saucepan over medium heat. Add the onion, ginger, and mustard seeds and cook, stirring, until softened, 2 to 3 minutes.
Add the chopped mangos, sugar, golden raisins, vinegar, and 1 teaspoon salt. Simmer, stirring, until the liquid is slightly thickened, 8 to 10 minutes; let cool. Refrigerate up to 2 weeks.