1) To make the crust: Whisk together the dry ingredients.
2) Work in the butter until the mixture is unevenly crumbly. Add the cheese and water, mixing until the dough is cohesive.
3) Divide the dough in half, pat each half into a disk, wrap in plastic, and chill for at least 30 minutes.
4) To make the filling: Sauté the onion and mushrooms in the butter until softened.
5) Add the spinach, salt, and pepper, and cook until warmed through. Cool to room temperature.
6) Preheat the oven to 400°F. Grease a baking sheet, or line with parchment.
7) Roll each piece of dough 1/4" thick. Cut out 10 rectangles (about 4" x 5"); a 5-wheel accordion cutter or pastry wheel both work well here.
8) Put 1/3 cup filling and 1 1/2 tablespoons cheddar cheese onto five of the rectangles. Brush a thin coating of water along the edge of each rectangle.
9) Stretch an unfilled rectangle over the filling, and crimp the edges of the pie with the tines of a fork to seal. Transfer to the baking sheets.
10) Bake the pies for 18 to 22 minutes, until golden.
11) Remove them from the oven, and serve warm.
Yield: 5 pocket pies.