Add the flour, butter and salt to a large bowl.. Using your fingertips (or a pastry cutter) rub the butter and flour until it resembles breadcrumbs. Add the Parmesan cheese. Drizzle in the water and mix using a fork until the it starts to come together and holds together when pressed in your hand. You may not need all of the water.
Wrap the dough in plastic wrap and refrigerate for 1 hour or overnight.
To make the pastry without a food processor, add the flour, butter, salt and Parmesan cheese to a large mixing bowl. Use your fingers to rub the butter into the flour, until it resembles breadcrumbs. Mix in the lemon zest. Use a fork while drizzling in the water. When the dough starts to come together, use your hands to form into a ball. Wrap in plastic wrap and refrigerate for 1 hour or overnight.
Dust a clean work surface with flour.
Preheat oven to 400° F/200°C.
Roll the pastry onto a floured surface into a 12 inch circle. Transfer the pastry to a 10 inch round baking pan or low sided cast iron skillet.
Cover the pastry evenly with the gruyere cheese leaving a 1 inch border all around. Arrange the tomatoes on top of the cheese, also leaving a 1 inch border.
Combine the basil, rosemary and pepper in a small bowl. Brush the tops of the tomatoes with the olive oil then sprinkle the herb mixture over the tomatoes.
Fold the border of the pastry in to cover the edge of the tomatoes all around. Brush the pastry edges with the egg wash. Bake for 20-25 minutes until the dough is golden.
Remove from the oven and allow to cool in the pan for 10 minutes, slice and serve.