Heat the pan and add olive oil. Then, place your beef into the pan. Season the beef with salt and pepper. Cook until brown.
Finely chop 50 g of shallots and 150 g cremini mushrooms. Fry them in the same pan, where the beef was cooked. Season with salt and pepper, add 20 g of finely chopped parsley.
On a work surface, roll 1 sheet of puff pastry into a 35-by-25-cm rectangle. Put teaspoon-sized mounds of the mushroom mixture on the pastry, evenly spacing them in 4 rows of 3. Top each mushroom mound with a piece of beef. Brush the tops of the beef with the Dijon mustard. With a sharp knife, cut the pastry into even squares around the fillings.
Then, make tightly sealed packets.
Crack one egg into the bowl and whisk it. Then, brush your packets with egg wash.
Place your packed beef on the tray with baking paper. Place the beef into the oven at 230 degrees C for 40 minutes.