Heat 1/2 tablespoon of oil in a heavy small skillet over high heat for 2 minutes. Brush tuna with 1/2 tablespoon olive oil; sprinkle with salt and pepper. Place in skillet and sear until slightly brown outside and almost opaque in center, about 2 minutes per side. Cool tuna; dice finely. Combine tuna, remaining 3 tablespoons of oil and all remaining ingredients in medium bowl. Using a fork, mix gently and evenly. Season tartare to taste with salt and pepper and chill. Serve with chips or over a small corn tostada.