Tuna Tacos with Onions

Name: Tuna Tacos with Onions
Ingredients: 3 tablespoons olive oil; 2 medium red onions, thinly sliced; Salt; 1 teaspoon chipotle powder or hot paprika; 1 teaspoon dried oregano; 1 tablespoon red wine vinegar; 1 ripe mango—peeled, pitted and cut into 1/4-inch dice; 1 medium cucumber—peeled, seeded and cut into 1/4-inch dice; 1 serrano or jalapeño chile, seeded and very finely chopped; 2 tablespoons fresh lime juice; 1 tablespoon chopped cilantro; One 1-pound tuna steak (1 inch thick); Eight 6-inch corn tortillas