Heat butter in skillet or braising pan over high heat. Add garlic and mushrooms and sauté for 8 to 10 minutes or until tender. Transfer to bowl.  Reserve 1/3 cup of mushrooms for use in gravy. Add spinach to pan and sauté over high heat for 3 to 4 minutes or until moisture has evaporated. Remove from heat. Reserve 1/3 cup spinach for use in gravy. Add cheese, walnuts and parsley to mushroom/spinach mixture and mix thoroughly.

 

Place pastry squares on floured work surface. Place 1 chicken breast filet on each pastry and tuck narrow end of chicken under itself to form a square. Top each chicken breast filet with 1/2 cup mushroom/spinach mixture. Fold opposite corners of each pastry square over the filling to meet at the center and pinch together to seal. Pinch remaining edges to seal. 

 

Transfer to parchment-lined sheet pans. Brush each pastry square with egg and bake in preheated conventional oven at 400 degrees F for 40 to 45 minutes or until pastry is golden and chicken is no longer pink (if using convection oven, bake at 350 degrees F for 18 to 20 minutes). Add reserved mushrooms and spinach to chicken gravy, mix thoroughly and keep warm. Spread 1/3 cup gravy on plate and top with 1 pastry square. Garnish with 2 tsp parsley.