For the pie dough:
1. Mix dry ingredients together in a medium sized bowl. Cut in butter until it resembles coarse crumbs.
2. Add water and vinegar one tablespoon at a time, slowly incorporating until mixture starts to form a shaggy mass. Pour onto lightly floured surface and fold gently until it all comes together. Wrap tightly in plastic wrap and chill at least 30 minutes before rolling out.
For the green tomato filling:
1. Toss diced tomatoes with sugar, cornstarch, and spices. Allow to sit for at least 15 minutes, until tomatoes have released their juices.
2. Strain tomato juices into a small saucepan and simmer until thickened, about 1-2 minutes. Toss thickened juice back in with tomato mixture. Allow to cool.
3. In the meantime, roll out pie dough and cut into small squares or rounds. I used a 3" drinking glass to cut circles. Divide pie circles onto 2 sheet pans lined with parchment paper. Cut little slits or a small 'X' into half of your pie rounds - these will be the tops. Chill pie dough in the refrigerator for about 20 minutes before filling.
1. Remove pie dough from refrigerator and scoop a tablespoon or two of tomato filling onto half of your dough rounds.
2. Whisk egg and water together in a small bowl.
3. Brush edges of pie rounds lightly with egg wash. Place top rounds over filling, pressing all around the edges to seal. Use a fork to crimp the edges together around the sides.
For the topping:
1. Combine cinnamon, sugar, and salt.
2. Brush tops of pies with more egg wash and sprinkle heavily with cinnamon mixture.
3. Chill pies again for another 20-30 minutes before baking.
4. Preheat oven to 375° F. Bake pies until crust is golden and filling is bubbling, about 25-30 minutes.