In a large skillet, heat 2 tablespoons of the olive oil. Add the shrimp, season with salt and pepper and cook over moderately high heat until barely cooked, about 2 minutes. Using a slotted spoon, transfer the shrimp to a medium heatproof bowl. Add the onion and red pepper to the skillet, cover and cook over moderate heat, stirring a few times, until softened, about 5 minutes. Transfer to the bowl. Heat the remaining 1 tablespoon of olive oil in the skillet. Add the garlic and cook until fragrant, about 1 minute. Add the cumin, turmeric and oregano and cook, stirring, for 30 seconds. Add the red wine vinegar and fish stock and boil over high heat until reduced to 1 cup, about 4 minutes. Stir in 1/2 teaspoon of salt until dissolved. Pour the marinade over the shrimp and vegetables in the bowl and stir well. Let cool to room temperature. Stir in the cilantro. Cover the shrimp and refrigerate for at least 6 hours or overnight.