Cut fish into 1 1/2-inch cubes, reserving all skin, trimmings and bones. Peel and devein shrimp, reserving shells. Set aside fish and shrimp.
In large saucepan over medium heat, bring 3/4 cup water, fish skin, trimmings and bones, shrimp shells, celery, wine and bay leaves to boil. Add mussels; cover and cook just until mussels open, about 7 minutes. Discard any that do not open. Using slotted spoon, transfer mussels to bowl; let cool for 5 minutes. Remove mussels from shells and transfer to clean bowl.
Meanwhile, strain liquid through fine sieve into clean saucepan and bring to boil. Add shrimp; reduce heat, cover and simmer until pink, about 2 minutes. Using slotted spoon, transfer to bowl with mussels. Return liquid to boil. Add fish; reduce heat and simmer until fish flakes easily when tested with fork, about 4 minutes. Using slotted spoon, transfer to bowl with shrimp and mussels. Pour liquid into measuring cup, adding water if necessary to make 2 1/4 cups.
In saucepan, melt butter over medium heat; cook onions and mushrooms until softened, about 6 minutes. Stir in flour, ginger, nutmeg, salt and pepper; cook, stirring constantly, for 1 minute. Gradually stir in liquid. Bring to boil, stirring constantly; cook until thickened and reduced slightly, about 5 minutes. Stir in cream; bring to boil and remove from heat. Stir in parsley and thyme.
Gently spread seafood mixture evenly in 13- x 9-inch glass baking dish; cover with sauce. (Make-ahead: Let cool for 30 minutes. Refrigerate until cold. Place plastic wrap directly on surface and refrigerate for up to 1 day. Or overwrap with heavy-duty foil and freeze for up to 2 weeks. Thaw in refrigerator for 24 hours. Add 10 minutes to baking time.)
On lightly floured surface and reserving 1-inch square of pastry, roll out remaining pastry to fit baking dish. Brush rim of baking dish with water. Cover with pastry, pressing down edges and crimping if desired. Cut steam vents in top. Roll out reserved pastry to 1/4-inch thickness; make cutout of fish. Whisk egg yolk with 1 tbsp water; brush over underside of fish. Place in centre of pie. Brush remaining egg yolk all over top of pastry. Bake in 400°F oven until pastry is golden and crisp on top, about 40 minutes.