Warm oil in a large saucepan over medium-high heat. Add bell pepper and onion and cook, stirring often, until softened, 3 to 5 minutes. Stir in mangoes, sugar, cumin, salt and pepper. Reduce heat to medium, partially cover and cook, stirring often, until mangoes have completely softened, 20 to 30 minutes. Remove from heat and stir in lime juice. Let cool. Cover and chill until ready to serve. Just before serving, stir in mint. Taste; adjust seasoning, if desired.