Oven Baked Borek with Mustard Greens, Feta, and Walnuts

Last summer, while vacationing with my husband at my family's house on the Aegean coast of Turkey, I went to market. It is the market I usually go to when there, the Saturday food market in Turgutreis, and I'll fill up with bags of tomatoes, eggplant, green peppers, garlic, cucumbers, parsley, apricots, peaches, grapes; the usual suspects. This time, I knew I would be forced to push my culinary boundaries when I came across a woman sitting in front of overflowing bags of mysterious green herbs.