For the pastry: Preheat the oven to 200 degrees C/gas 6 (390 degrees F).
Mix the flour and salt in a bowl. Gradually add enough of the water to form a dough. On a lightly floured work surface, roll the dough out into a 20-by-30-centimeter (8-by-12 inch) rectangle.
Scatter half of the grated frozen butter and lard over the pastry. Fold down the top third of the pastry and then fold up the bottom third, as if folding a letter. Turn the folded dough 90 degrees and roll it out into a rectangle again. Repeat the process: adding the remaining frozen butter and lard, and folding as before. Roll out and repeat 2 more turns with the dough to create the pastry layers. Wrap the dough in plastic wrap and leave to rest in the fridge for 30 minutes before using.
For the filling: On a lightly floured work surface, roll out the pastry into a 25-by-35-centimeter (10-by-14-inch) rectangle. Place the spinach on the base of half of the pastry, leaving a 1 1/2-centimeter (1/2-inch) gap from the edge. Add a layer of chopped peppers, cover with the grated cheese, and top with a layer of Iberico ham. Brush the edges of the pastry with beaten egg. Fold the pastry in half to cover the filling. Press the edges to seal. Brush the top with beaten egg and score a pattern on the top with a knife.
Place on a baking sheet and bake until the pastry is crisp and a rich golden brown, 30 to 35 minutes. Eat warm or cold with a salad.