Chop the blonde onion and put it in a pan with oil. Light a low flame and let it brown. Add the sausage with the gut removed, shell it well with the prongs of a wooden fork and bring it to complete cooking. Adjust the pepper and, if necessary, salt.
Cut the mild Provolone Valpadana P.D.O. into cubes. Unroll the puff pastry on the work surface and cut it with a round pastry cup to obtain an even number of circles, on half of which you will place the sausage and the mild Provolone Valpadana P.D.O. in the centre.
Brush the edges around the filling with water and cover with the remaining pasta circles. Seal well with the tines of a fork.
Place the medallions on a baking tray covered with baking paper and transfer them to the freezer 15 minutes, in this way the puff pastry will deform less during cooking.
Brush with beaten egg, sprinkle with poppy seeds and bake at 180° until golden brown, about 20 minutes.