In the bowl of a standing mixer fitted with a paddle, mix the flour and the salt. Add the egg, oil, and seltzer, and mix to form a soft dough. Turn out the dough and knead, by hand, for 1 minute, until the dough is elastic.
Divide the dough into 4 pieces, form into balls, cover in plastic, and let rest in the refrigerator for 1 hour.
Preheat oven to 350 degrees F.
When chilled, roll the ball into a square. Trim the edges, and brush with melted butter. Fold the dough in half, and brush with butter. Fold the dough in half again, to make a square. Place 1/2 cup Spinach-Cheese Filling in the center. Sprinkle with some string cheese. Fold up the corners like a half open envelope. Repeat with remaining ingredients. Bake for 25 minutes.
For the Spinach-Cheese Filling: Mix together the cream cheese, hazelnuts, garlic, cilantro, coriander, cayenne, fenugreek, salt, and black pepper. Thoroughly drain the spinach and chop. Mix into the mixture, then fold in the feta. Set aside until ready to use.