Place egg yolk, water, lemon juice, and mustard in the bottom of an immersion blender cup. Pour oil on top and allow to settle for 15 seconds. Place head of immersion blender at bottom of cup and switch it on. As mayonnaise forms, slowly tilt and lift the head of the immersion blender until all oil is emulsified. Season mayonnaise to taste with salt. Store in a sealed container in the refrigerator for up to two weeks.