In a large bowl, combine flour, salt, and cheese. Add butter and using your fingers or a pastry cutter, work the butter into the flour until small, pea-sized clumps form. Add water, 2 tablespoons at a time and work into flour until dough forms. Use only as much water as needed for dough to form together.
Form dough into a disk and wrap tightly with plastic wrap. Refrigerate while you make filling.
For the filling and assembling the hand pie;
For the filling: In a large skillet, cook pancetta over medium-high heat until fat renders and pancetta starts to get crispy, about 5 minutes. Using a slotted spoon, transfer pancetta to a paper towel lined paper to drain. Add leeks to pan with pancetta drippings and cook until they soften, about 3 minutes. Stir in spinach and sage and cook until spinach is wilted, about 3 minutes. Sprinkle flour over vegetables and stir to coat. Add in broth and cook until mixture thickens, about 2 minutes. Remove from heat and stir in pancetta. Season with salt and pepper to taste. Let cool.
For the hand pie: Preheat oven to 350F.
On a lightly floured surface, roll out rough to about ¼ - ½ inch thick. Using a small bowl or cookie cutter, cut out 12 circles about 4½ inches in diameter. You may need to re-roll scraps to get all the circles.
Place six of the circles on a baking sheet. Scoop about ¼ cup of spinach and pancetta filling into the center of each circle, staying about ½ an inch away from the edges.
Using your finger or a small pastry brush, brush the edges of the circles with egg wash. Place remaining six circles on top and pinch edges shut. Optionally, use a fork to crimp the edges shut.
Brush tops of hand pies with remaining egg wash, then cut small slits in the tops of each pie. Bake for 30-35 minutes, until pies are golden brown. Serve warm or at room temperature.