Bake the pastry shells as directed on the package, and keep warm.
Drain the liquid from the escargots. Place butter, snails, and mushrooms in a sauté pan over medium heat and cook until the butter has melted, stirring occasionally.
Add the cream and 2 tablespoons of the parsley to the pan, stirring to mix thoroughly, simmer until the sauce has thickened slightly, stirring occasionally, about 20 minutes.
Spoon the snails and mushrooms into the pastry shells and pour the sauce over them. Garnish with remaining 2 tablespoons parsley.