- 1 tablespoon unsalted butter
- 1/4 cup finely chopped onion
- 1 pound large shrimp (21 to 25 per pound in shell), peeled, deveined, and coarsely chopped
- 2 tablespoons medium-dry Sherry
- 1 tablespoon chopped fresh tarragon
- 1/2 teaspoon salt
- 1/4 teaspoon black pepper
- 1 (17 1/4-ounce) package frozen puff pastry sheets, thawed
- All-purpose flour for dusting
- 1 large egg
- 1 tablespoon milk
- Special equipment: parchment paper
- Put oven racks in upper and lower thirds of oven and preheat oven to 400°F. Line 2 large baking sheets with parchment paper.
- Heat butter in an 8-inch skillet over moderately low heat until foam subsides, then cook onion, stirring occasionally, until softened but not browned, about 2 minutes. Transfer to a bowl to cool.
- When onion is cool, add shrimp, Sherry, tarragon, salt, and pepper and stir until combined well.
- Roll out 1 sheet of pastry on a lightly floured surface into a 12-inch square. Cut into thirds in one direction, then into fourths in the other direction to make 12 (3- by 4-inch) rectangles. Cut each rectangle in half diagonally to form 2 triangles. Place 1 triangle on a work surface with longest edge nearest you, then put 1 teaspoon shrimp filling in center of triangle. Bring bottom corner of shortest side up to top point, then wrap remaining corner around resulting cone. Pinch to seal, then transfer to a lined baking sheet. Form more cornets in same manner, then repeat with remaining sheet of pastry. Chill first sheet of cornets, loosely covered, while making second.
- Whisk together egg and milk and lightly brush some of egg wash over top of pastries.
- Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until golden, 18 to 20 minutes total.
- Cool cornets on sheets on racks 5 minutes, then gently loosen from parchment with a spatula. Serve warm.