- eight 17- by 12-inch phyllo sheets
- 5 1/2 tablespoons unsalted butter
- 1 1/2 tablespoons all-purpose flour
- 1/2 cup whole milk
- 1/2 cup heavy cream
- 2 smoked trout fillets (about 1/2 pound total)
- 6 large eggs
- 1 1/2 teaspoons drained bottled horseradish
- 2 tablespoons fresh dill leaves
- available at fish markets, the deli counter of many supermarkets
- Cover phyllo stack with overlapping sheets of plastic wrap and then a damp kitchen towel.
- Melt 4 tablespoons butter. Arrange 1 phyllo sheet on a work surface and brush with some melted butter. Arrange another sheet to overlap the first and form a 17-inch square and brush with butter. Continue layering with remaining 6 sheets and butter. Drape phyllo stack over a 10-inch tart pan with a removable bottom and fit phyllo into pan. Crumple overhang against inside edge of rim to create a ragged edge that stands about 1 inch above rim. Chill shell, loosely covered, until firm, at least 3 hours.
- Preheat oven to 375°F.
- Prick bottom of shell all over with a fork and bake in middle of oven until golden, about 15 minutes. Cool shell in pan on a rack. Shell may be made 1 day ahead and kept, covered, at cool room temperature.
- In a saucepan melt remaining 1 1/2 tablespoons butter over moderately low heat and whisk in flour. Cook roux, stirring, 3 minutes and whisk in milk and cream. Bring mixture to a boil, whisking constantly, and simmer, whisking occasionally, 3 minutes. Season mixture with salt and pepper and cool.
- Discard skin and bones from trout and break fish into small pieces. Separate eggs. In a food processor pulse together milk mixture, trout, yolks, horseradish, and dill until smooth and transfer to a large bowl.
- In another large bowl with an electric mixer beat whites with a pinch salt until they just hold stiff peaks. Stir one fourth whites into trout mixture to lighten and fold in remaining whites gently but thoroughly.
- Pour soufflé mixture into shell and run tip of a knife around edge of soufflé to aid rising. Bake soufflé on a baking sheet in lower third of oven until puffed and golden brown, about 25 minutes. Serve soufflé immediately.