Place the capers, mustard, anchovies, egg and 2 tablespoons lemon juice in the bottom of an immersion blender cup. Pour 1/2 cup vegetable oil on top and allow to settle for 15 seconds. Place the head of the immersion blender at the bottom of the cup and switch it on. As mayonnaise forms, slowly tilt and lift the head of the immersion blender until all the oil is emulsified. Add the tuna and remaining 2 tablespoons lemon juice and blend until thoroughly incorporated into a smooth sauce.
Transfer the mayonnaise to a large mixing bowl and slowly drizzle in the olive oil, whisking until emulsified. Season with salt and pepper. |