In a large skillet heat 2 tablespoons of olive oil.
Add the onions, season with salt and cook over medium heat for about 8 minutes, until softened and browned. Add the paprika and cook for 30 seconds. Add the vinegar and the oregano. Cook for a minute and transfer to a plate.
Using a bowl, combine the cucumber, the mango, Serrano, cilantro and lime juice. Add a little salt, mix well, reserve.
Rub the tuna with 1 tablespoon of olive oil; season with salt. Using a grill pan over high heat, grill the tuna until lighted charred, about 3 minutes each side. Remove from pan.
Grill the tortillas for about 15 seconds, or until soft and hot.
Thinly slice the tuna.
Arrange all ingredients in a platter, so you can prepare the tacos with the onions, the mango salsa, and fresh cilantro.
To keep the tortillas warm, wrap them in aluminum foil right after grilling.