Bake the puff pastry shells according to the package directions until deeply golden and hollow out the soft centers.
Melt the butter in a large skillet over medium-high heat and cook the mushrooms and onion for 7 minutes until browned and dry.
Stir in the flour for 1 minute, then whisk in the broth and cream and simmer for 5 minutes until smooth and thickened.
Add the chicken, peas, pimentos, salt and pepper and cook for 4 minutes until the filling reaches 165°F.
Spoon the hot chicken mixture into the pastry shells immediately before serving so the bottoms stay crisp.