Add flour, salt and pepper to a food processor and pulse a couple of times to mix. Add butter gradually and pulse until the largest pieces of butter are about the size of large peas. Then add ice water, a tablespoon at a time, pulsing once or twice after each addition until the dough just barely begins to hold together.

 Carefully empty the crumbly dough mixture from the food processor on to a clean, dry surface. Gather the mixture into a mound, add walnuts and use your hands to knead each mound just enough to incorporate walnuts and form into a disk. Wrap disk and refrigerate for one hour or up to 2 days.

Add oil to a skillet over medium heat. When the oil is hot, add leeks and cook until softened, about 5 minutes. Add Swiss chard and balsamic and continue to cook for another 2-3 minutes. Season with salt and pepper and remove from heat. Allow mixture to cool completely and discard any excess liquid in the pan before moving on to the next step.

Heat oven to 350 degrees. Remove dough from the refrigerator and let sit at room temperature for 5-10 minutes to soften and make rolling out easier. Roll out with a rolling pin on a lightly floured surface to a 12-inch circle about 1/8 of an inch thick. Transfer to a silpat or parchment lined baking sheet.

Spread a thin layer of Dijon mustard in the center of the circle, leaving about 1 1/2 inches around the edge. Top with Pecorino Romano, then top with vegetable mixture. Bring up the edges of the dough and fold over to form a crust. Brush the crust lightly with beaten egg and bake for 20-25 minutes. Check the bottom of the crust by lifting it up gently with a spatula to make sure has cooked and hardened before removing from the oven. Let cool slightly and cut into wedges to serve.