In a food processor, combine the flour, Parmesan, salt, and pepper. Add the butter cubes and pulse machine on and off until the mixture looks like coarse cornmeal. Slowly add 1/4 cup of ice cold water and pulse until the dough is crumbly but comes together when squeezed. Add additional water if the dough is too dry. Turn the dough out onto a lightly floured surface. Form into a flat disk and wrap in plastic wrap. Refrigerate the dough for at least one hour or up to three days.
In a large sauté pan over medium heat, add the olive oil and 4 tablespoons of unsalted butter. Once the butter is melted, add the onions and leeks. Sauté for about 10-15 minutes or until onions and leeks are just golden and soft. Add the kale, swiss chard, and garlic . Sauté until the greens are wilted, about 5-7 minutes. Season with salt and pepper. Set aside.
In a large sauce pan over medium heat, melt the remaining 4 tablespoons of butter. Sprinkle the flour over the melted butter and whisk for about 1 minute until a paste forms. Slowly add the milk and continue to whisk the mixture until it thickens, about 5 minutes. Remove from the heat and whisk in one cup of Parmesan and the Gruyère cheese. Whisk until the cheese is melted and incorporated. Stir the greens into the mixture until well mixed.
Preheat the oven to 375 F. Take the dough out of the refrigerator and allow it to soften slightly for about 5 minutes. Meanwhile, lightly flour a clean surface or nonstick pastry mat. Roll the dough into a 15-inch round. Transfer the dough to a parchment lined rimmed baking sheet. Pile the greens filing in the center of the dough leaving a 2-inch border around all the sides. Fold the border into the filling, pleating and gently pressing down as you go. In a small bowl, whisk together the egg and 1 teaspoon of water. Brush the dough with the egg wash and sprinkle the remaining 1/4 cup grated Parmesan cheese over the crust of the dough. Bake for about 45 minutes or until crust is golden and the mixture is bubbling. Cool slightly and then zest fresh lemon peel over the top. Cut into wedges to serve.