"Most devout Russian Orthodox families ate no meat during Lent, and this salmon dish was enjoyed especially to celebrate on Easter. This recipe can also be made 'family-style' by using one whole 24-ounce salmon fillet to make one large Koulibiaka."
To make the rice pilaf, heat 2 tablespoons butter in a saucepan over medium heat. Stir in 2 1/2 tablespoons of onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Add the rice and stir to coat with butter. Pour in 1 cup chicken broth and bring to a boil. Reduce the heat to low, cover, and simmer until the rice is fluffy and all of the liquid is absorbed, about 20 minutes. Season with salt and pepper to taste. Set the rice aside.
For sauce, melt 1/4 cup of butter in a saucepan over medium heat until a pinch of flour sprinkled into the butter will just begin to bubble. Whisk in the flour to form a thick paste the consistency of cake frosting. Continue cooking, whisking constantly, until the flour has turned golden, and begins to smell like toasted bread, about 20 minutes. Whisk in 2 cups chicken broth and simmer until thickened and bubbly, about 5 minutes. Pour the sauce into the rice pilaf, reserving about 1/4 of it for serving, if desired.
Heat another 1/4 cup of butter in a skillet over medium heat. Stir in the mushrooms and cook until soft, about 5 minutes; add the 1 1/2 cups onion. Cook and stir until the onion has softened and turned translucent, about 8 minutes. Combine the onions and mushrooms with the rice mixture and allow it to cool slightly. Stir in the lemon juice, chopped hard-cooked eggs, and fresh dill.
Preheat an oven to 400 degrees F (200 degrees C). Line a baking sheet with buttered parchment paper.
Bring a large pot of lightly salted water to a boil. Add the cabbage leaves and cook, uncovered, until softened, about 2 minutes. Drain in a colander, then immediately immerse in ice water for several minutes to stop the cooking process. Once the leaves are cold, drain well, and blot dry with paper towels or a clean kitchen towel.
On a lightly floured surface, roll out the puff pastry to a 1/4-inch thickness. Cut the pastry into 6 rectangular shapes; they should be three times wider than the salmon fillets, and 2 inches longer. Lay 2 cabbage leaves on each rectangle and top each with a spoonful of the rice mixture. Place each fillet "skin-side" up on top of the rice, and season with salt and black pepper. Top the fillets with another spoonful of rice; fold the cabbage leaves over the fillets to encase them. Center the folded leaves on the puff pastry.
Fold the short ends of the pastry in, and then the sides together to make a packet; pinch along the seams to seal. Place the pastries onto the parchment-lined baking sheet seam sides down. Cut a small slit in the center of each pastry to act as a steam vent. Beat the egg and 1 tablespoon water together in a bowl; brush the pastries with the egg wash.
Bake in the preheated oven until the pastries are deep golden brown, about 15 minutes. The internal temperature of the salmon should reach 135 degrees F (57 degrees C).