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
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.