For filling

1 small tomato, chopped
1 small onion, finely chopped
2 tablespoons unsalted butter
1/8 teaspoon salt
2 large white mushrooms, finely chopped
1/2 cup leftover gravy

1 1/4 cups finely chopped cooked turkey
1 tablespoon chopped fresh flat-leaf parsley

For cream puff

3/4 cup water
6 tablespoons cold unsalted butter, cut into small pieces
1/2 teaspoon salt
3/4 cup all-purpose flour
4 large eggs, at room temperature for 30 minutes
3 oz Swiss cheese, cut into 1/4-inch dice
2 tablespoons finely grated Parmigiano-Reggiano

1/2 teaspoon dry mustard powder


Make filling:

Cook onion in butter with salt in a 10-inch heavy skillet over moderate heat, stirring occasionally, until golden, about 4 minutes. Add mushrooms and cook, stirring occasionally, until mushrooms begin to brown, about 3 minutes. Stir in gravy and bring to a boil over moderate heat, stirring constantly. Reduce heat and simmer, stirring, 1 minute (sauce will be thick). Remove from heat and stir in turkey, parsley, chopped tomato, and pepper to taste.

Make cream puff dough:

Put oven rack in middle position and preheat oven to 375°F.

Bring water to a boil with butter and salt in a 1 1/2- to 2-quart heavy saucepan. Reduce heat to low and add flour all at once, then stir vigorously with a wooden spoon until dough forms a ball and pulls away from sides of pan, 1 to 2 minutes.

Remove from heat and stir vigorously for 1 minute more to release steam, then beat in 3 eggs, 1 at a time, beating until incorporated after each addition. Stir in Swiss cheese, 1 tablespoon parmesan, and mustard, then spread dough with dampened hands into a buttered 9-inch pie plate. Create a 6-inch-wide hole in center by pulling dough toward edge of pie plate, then mound filling in hole. Push dough up sides of filling, leaving top of filling exposed. Lightly beat remaining egg and brush some over dough (reserve remainder for another use), then sprinkle with remaining tablespoon parmesan.

Bake pie until dough is puffed and golden, about 40 minutes, then cut 4 slits with a small knife around side of dough to release steam and bake 10 minutes more. Let pie stand 5 minutes before serving.