Preheat the oven to 350F, and grease a 9x13-inch baking dish with butter and a little flour.

In a large mixing bowl, combine the flour and salt. Add in the butter pieces, and break them up using your fingers until they are pea-sized. Make a well in the center and add the water. Knead the dough until it just comes together. 

Take ⅔ the dough and form into a flat rectangle. Wrap in plastic. Do the same with the remaining dough, and transfer to the refrigerator to chill.

In a large mixing bowl, add the spinach, cheeses, two eggs, and salt and mix well to combine. Set aside.

Lightly flour a surface and roll out the larger portion of dough. It should be large enough to cover the bottom and inner sides of the baking dish. Press the dough gently into the baking dish. Spread the filling in an even layer, and roll out the remaining dough. It should be large enough to cover the top of the baking dish. Press out any air bubbles and cut a few slits into the dough with a knife to allow steam to escape during baking. 

Fold the excess dough over the edges and pinch together to seal. Brush with the egg yolk, transfer to the oven, and bake until the top is evenly browned and the dough is cooked through, about 40-50 minutes,