Prepare puff pastry shells according to package directions. While oven is preheating for shells, have all other ingredients on the counter and ready to go, as you must work quickly to keep everything hot. You can make the filling while the shells bake and keep it warm.
Clean mushrooms, trim stems, and slice. Heat butter in saute pan over medium heat and add mushrooms. Cook, stirring frequently, until liquid has evaporated and mushrooms are browned.
While mushrooms are cooking, stir together sour cream, Parmesan, bourbon, flour, salt, and pepper together in a small mixing bowl; set aside. When mushrooms have cooked, reduce heat to low and add sour cream mixture. Stir constantly until mixture is thickened and blended. *Note: start with the ⅔ cup sour cream, but since various brands of sour cream have different thicknesses, you might need to add a little more if it is too thick.
When shells are done, and the middle part has been removed as directed on package, fill each shell immediately with the mushroom mixture, dividing evenly. Garnish with watercress sprigs. Serve while hot.
Recipe by The Culinary Cellar at https://theculinarycellar.com/big-book-sides/