Preheat oven to 180°C (350°F) and place pastry shells on a baking sheet to warm for 10 minutes.
In a sauté pan over medium heat, melt 10g of butter and cook sliced mushrooms until golden and moisture has evaporated. Set aside.
In the same pan, sauté shrimp for 2-3 minutes until opaque and pink. Remove from heat and set aside.
In a heavy-bottomed saucepan, melt the remaining 30g of butter over medium-low heat. Whisk in flour and cook for 2 minutes without browning.
Slowly whisk in the fish stock, ensuring no lumps form. Simmer for 5-7 minutes until the sauce thickens.
In a small bowl, whisk together the heavy cream and egg yolk. Temper the mixture by adding a spoonful of hot sauce, then stir the cream mixture back into the saucepan.
Add the mushrooms and shrimp to the sauce. Stir in lemon juice, salt, and white pepper. Heat gently for 2 minutes; do not allow the sauce to boil.
Ladle the hot filling into the warm pastry shells. Garnish with chopped parsley and serve immediately.