Begin by seasoning the chicken breasts with salt and pepper. Sauté them in a hot pan with a bit of olive oil until they are golden brown and cooked through, about 6-7 minutes per side. Once done, remove them from the pan and let them rest.

In the same pan, add finely chopped shallots and sauté until translucent. Stir in the mustard and cook for another minute before adding the heavy cream. Simmer the sauce until it thickens slightly, about 3-5 minutes. Adjust the seasoning with salt and pepper.

Roll out the puff pastry on a floured surface. Cut it into squares large enough to encase each chicken breast. Place a chicken breast in the center of each square and spoon some mustard cream sauce over it. Fold the pastry over the chicken and seal the edges by pinching them together.

Preheat your oven to 400°F (200°C). Place the assembled pastries on a baking sheet lined with parchment paper. Brush the tops with the egg wash and bake for 25-30 minutes or until they are golden brown and puffed up.

Once baked, let the pastries cool for a few minutes before serving. Drizzle with additional mustard cream sauce if desired, and garnish with fresh herbs.