1) In a food processor combine the flour, and 1/2 tsp salt. Pulse to combine. Add the butter and pulse until the butter is finely chopped and the mixture resembles a coarse meal. In a small bowl combine the lemon juice and mascarpone. Add the mascarpone mixture to the food processor and pulse a few times. Add the ice water and run the machine just until the mixture is moist and crumbly, but does not form a ball. Do not over mix. Turn the dough out onto a sheet of cling film. Press into a disk, cover, and refrigerate for 20 mins.
2) Meanwhile, preheat the oven to 200C/Gas 6. Cook the pancetta in a small saute pan over medium heat until golden and crisp, about 4 mins. Add the herbs and stir. Transfer the pancetta, herbs, and the pan juices to a medium bowl. Add the leftover vegetables, grated cheeses, the remaining 1/2 tsp salt, and pepper. Toss to combine and set aside.
3) Place the chilled dough on parchment paper. Roll the dough out into a 30cm wide circle about 1/2cm thick. Spread the vegetables out in the centre of the dough leaving a 5cm border. Sprinkle the vegetable mixture with Parmesan. Fold the border up and over the vegetables forming a crust. Using a pastry brush, brush some of the egg over the crust. Transfer the parchment paper and crostata to a baking sheet. Bake until the crust is golden, about 25 mins. Slice and serve.