Pre-heat oven to 375 degrees F. Melt some butter in the cast iron skillet over medium heat so the peaches don't stick. Set aside.
Toss frozen and thawed (or fresh) peaches with flour, sugar, salt, cinnamon, grated lemon zest and juice from 1 lemon. Arrange in prepared cast iron skillet.
Then make the topping: combine flour, sugar, baking powder, and salt in a medium bow. In a separate smaller bowl, whisk together the Greek yogurt, melted butter, milk and vanilla. Add the Greek yogurt mixture to the flour mixture. Stir until just combined. You'll have a loose "dough".
About ¼ cup a time, scoop the dough on top of the peaches, spacing each mound of topping out. You'll have about 5-6 mounds of dough.
Brush the dough with a little milk and sprinkle the whole cobbler with 1 tablespoon of sugar (optional).
Bake in the pre-heated oven for 45 - 50 minutes. Tops of the cobbler should brown.
Let sit in the cast iron pan for about 10 minutes before serving. Serve warm.