These elegant tarts look as if they take far more expertise to produce than the simple effort that is required. Using purchased puff pastry is what makes them so easy and quick to assemble. Count on 15 minutes to make the filling, and five for cutting out the puff pastry squares. Both the squares and the filling can be prepared several hours ahead so that at serving time you will need only to spoon the asparagus/ mushroom melange onto the pastries, and then bake them for 20 minutes. Serve the tarts as a first course or offer then with a green salad for a summer lunch or light supper.