Preheat oven to 425F. Roll out cold puff pastry onto a baking sheet lined with parchment paper. Fold edges in; you're making a half inch frame around the rectangle. Add mustard -- spread evenly across the base. Be a bit generous -- when it cools down it mellows in taste. Then spread tomato sauce, then bread crumbs, then layer the tomatoes. Add salt and freshly ground pepper, and Italian herbs. Wrap in plastic wrap and return to fridge briefly to cool. OPTIONAL: for a shiny brown crust, paint the exposed puff pastry with egg wash (whisked egg with 2 teaspoons milk). Drizzle with olive oil. Bake at 425 for 20-30 mins.