Individual portions are easier to serve at dinnertime than a single large coulibiac, a Russian dish of puff pastry enveloping poached salmon, mushrooms, and rice.
Bring wine, 2 cups water, bay leaf, celery, and peppercorns to a simmer in a small, wide saucepan over medium-high. Add salmon, cover, reduce heat to low, and cook until slightly opaque but still pink on inside, about 3 minutes. Remove salmon; let cool slightly. Strain liquid through a fine-mesh sieve; reserve. Flake salmon into large pieces.
Melt 3 tablespoons butter in a medium saute pan over medium-high. Add leek and cook, stirring occasionally, until tender and lightly browned, about 5 minutes. Transfer to a large bowl. Melt 4 more tablespoons butter in pan; add mushrooms and cook, stirring occasionally, until well browned, about 5 minutes. Add to leeks.
Melt remaining 1 tablespoon butter in pan. Whisk in flour, stirring constantly, until mixture is foamy and light brown, about 1 minute. Gradually whisk in 1 cup reserved poaching liquid. Bring to a boil, reduce to a rapid simmer, and cook, whisking constantly, until slightly thickened, about 2 minutes. Remove from heat; let cool. Stir into leek-mushroom mixture. Add rice, dill, and salmon. Season with salt and pepper and stir to combine.
On a floured surface, roll each piece of puff pastry (dusting with flour as necessary) into a 12-by-16-inch rectangle, 1/8 inch thick. Cut out twelve 4-by-6-inch rectangles; transfer to 2 parchment-lined baking sheets. Use small cookie cutters to cut shapes from excess dough, if desired.
Preheat oven to 425 degrees, with racks in upper and lower thirds. Divide filling evenly among pastry rectangles (about 1/4 cup each), mounding in center of one short side. With a pastry brush, dampen borders with cold water. Fold tops of pastry over filling. Press edges; crimp to seal tightly with the tines of a fork. Place cutouts on top, if desired; adhere with cold water. Cover with plastic wrap; refrigerate until firm, 20 minutes.
Beat together yolk and cream; brush each coulibiac with mixture. Bake, rotating halfway through, until puffed and golden brown, about 30 minutes. Serve warm, with lemon wedges and sauce.
The filling can be made a day ahead and stored in an airtight container in the refrigerator. The puff pastry can be rolled, cut, and frozen a few days ahead; bring it to room temperature before filling.