Preheat oven to 375° F. Cover a large cookie sheet with aluminum foil. Spray with non-stick vegetable spray.
In a large skillet, heat olive oil. Sauté onions and garlic in oil over medium heat, periodically stirring, until onions are caramelized. This can take up to 15 minutes.
Spread a thin layer of mustard on puff pastry. Place a layer of pastrami on the mustard, covering the entire sheet, using the entire ½ pound. Place a layer of turkey on the pastrami, covering the entire sheet, using the entire ½ pound. Evenly spread the caramelized onions over the layer of turkey. Roll. Place on the prepared cookie sheet.
With a sharp knife, make several diagonal slits across the top of the roll. Whisk together the egg and water. Brush over the top of the roll. Bake for 45 minutes until top of roll is golden brown. Serve sliced with additional mustard and sautéed onions.
Recipe posted with permission from Kosher Everyday by Sharon Matten