My girlfriend got this at a Turkish supermarket in Berlin. It's quite spicy and tastes like garden cress or a bit like radish. I looked up cress varieties but couldn't find this particular leaf shape.
I think you have Baby Argula or Rocket. Check out this site for Turkish greens. http://www.turkish-cuisine.org/ingredients-7/ingredients-used-in-turkish-cuisine-66/vegetables-69.html