1. Preheat oven to 425 degrees, with racks in upper and lower thirds. Season cavities and surface of fish with salt. Place 1 pastry sheet flat on a work surface, and arrange 2 trout side by side, 2 inches apart, on top. With a paring knife, cut out an oval of pastry around each fish, with about 1/2-inch border on sides, leaving heads and tails sticking out from pointed ends of oval. Repeat with remaining pastry and fish. Set fish aside. Use knife to score pastry ovals in a crosshatch pattern.

2. Generously oil 2 rimmed baking sheets. Arrange 2 trout on each sheet, then place ovals over trout. Brush tops of pastry with egg. Bake, rotating sheets from top to bottom halfway through, until pastry is golden brown and fish are cooked through, 20 to 22 minutes.

3. Meanwhile, combine sorrel, mint, capers, and oil in the bowl of a food processor. Pulse until mixture is smooth. Transfer to a bowl; fold in creme fraiche.

4. Spread 1/4 cup sorrel sauce on each of 4 plates. Place a fish on top of sauce. Serve immediately, with remaining sauce alongside.