1) Preheat the oven to 180°C/ gas mark 4. Line two baking trays with greaseproof paper.
2) Place the marinara sauce in a medium saucepan over medium heat and cook until reduced to a thick, jam-like consistency, about 30 minutes. There should be 340 to 450g of reduced sauce. Let the sauce cool to room temperature, about 30 minutes. Stir in the mozzarella cheese and prosciutto.
3) Place 1 piece of pastry on a work surface. Roll the dough out gently until you can cut an 22 by 24-cm rectangle. Press the scraps into a small rectangle and roll it out to the same thickness as the other rectangle. Cut 3 rectangles from the large rectangle and 1 rectangle from the small rectangle. Each should measure 22 by 8-cm. Continue rolling and cutting the remaining pastry.
4) To form the tarts, place the rectangles on a work surface. Brush each rectangle with egg wash. Place a large spoonful of the marinara mixture in the middle of the bottom half of the rectangle. Fold the dough in half over the marinara mixture to make a 8 by 10-cm rectangle. Press the edges to seal. Using a scalloped ravioli cutter, trim the rectangles to make a decorative edge. Place the tarts on the baking sheets. Brush with egg wash and generously sprinkle with cheese. Bake until golden, about 20 to 25 minutes.