Preheat oven to 350 degrees F. Melt butter over low heat and saute onion until translucent. Mix in rice, mushrooms, dill, and lemon juice and let cool.
Lay out puff pastry, place rice mixture over half of pastry and place salmon on top. Season with salt and pepper, and cover with eggs. Fold over pastry and seal with beaten egg.
Crimp the edges of the kulebyaka. Brush with egg and poke a couple of holes in the pastry to let out steam. Bake for about 30 minutes until pastry is golden brown. Let cool and serve.