Thaw the puff pastry.
Add the spinach to gently boiling water and cook until it is just wilted. Drain and set aside in a medium bowl.
Heat ½ of the oil and saute the onion and garlic until soft. Add the cooked spinach and mix with the onions and garlic, heating for about 1 minute. Put the mixture back into a bowl and toss with cream cheese until it is well distributed and melted.
Heat the remaining oil and saute mushrooms (season with herbes de provence). Cook until the mushroom are cooked and releasing moisture. Add the chicken, salt and pepper to taste and cook until no longer pink. Let cool in the pan.
Place the puff pastry onto a lightly floured surface and roll it out to approx. 11" x 15". Gently lift it onto parchment paper.
Spread out the spinach mixture on the pastry leaving a 3 inch border around each edge. Do the same with the chicken and mushroom mixture. Brush the beaten egg around the perimeter of the borders.
First fold the short sides of the pastry inwards, then fold in the long sides forming a cylinder. Pinch the seams together and brush the top of the puff pastry with the remaining egg. Cut three slits in the top of the pastry and slide it with the parchment paper onto a baking tray with raised sides.
Preheat your oven to 350℉. Use the convection setting, if your oven is equipped.
Bake for approx. 45 minutes. The chicken wellington should be puffed and golden brown. Place on a cutting board and let stand for 5 minutes before slicing. Cutting in six, 2 inch slices (plus the ends) will easily serve 4 people.