Pro tip: Substitute cultivated mushrooms with oyster or wild mushrooms for enhanced flavor.
Clean the mushrooms, trimming the earthy stem bottoms. Slice caps into strips and dice stems. Peel and finely chop the shallots. Melt 20 g butter in a saucepan over medium heat. Sauté shallots and diced stems until browned. Add mushroom strips, chopped chives, cream, salt, and pepper. Simmer uncovered for 10 minutes.
Meanwhile, poach fresh salmon in simmering water lightly seasoned with lemon juice, salt, and pepper. Drain thoroughly and flake into pieces.
Roll out puff pastry on a floured surface to cut two discs slightly larger than a medium round springform pan. Brush pan bottom and sides with remaining melted butter. Line with one disc.
Crumble 1 slice smoked salmon on base. Layer flaked fresh salmon, half the mushrooms, remaining crumbled smoked salmon, then rest of mushrooms.
Top with second pastry disc, crimping edges to seal. Brush with egg yolk; cut a small chimney hole in center.
Bake at 200°C for 30 minutes until golden.
Source: Regional Cuisines of France.
