This is a poultry variation of the well known Beef Wellington.
Serves: 4 people. Preparation time: 90 minutes.
Thaw the pastry sheet at room temperature about 30 minutes.
Preheat your oven to 400 degrees.
Mix the egg and water. Mix the cream cheese and mustard together.
Sprinkle the chicken breasts with thyme (or other herbs), as well as salt and pepper. Melt 1 tablespoon of butter in a skillet. Add the chicken pieces and cook until brown, but not quite all the way through. Place the cooked chicken on a covered dish and refrigerate 15 minutes or more.
Add the other tablespoon of butter to the skillet. Add the sliced mushrooms and chopped onion and cook until tender and liquid has evaporated. Stir in the parsley. Add optional garlic and a little white wine to taste. Reduce the liquid again.
Unfold the thawed pastry sheet on a lightly floured surface. Roll into a 14x14 inch square and cut into four 7x7 inch squares. Spoon 1-3 tablespoons of mushroom mixture on the center of each square. Add one chicken piece (trimming if necessary). Spread 1-2 tablespoons of cheese/mustard mixture on each chicken breast. Use up all the mushroom mix and cheese/mustard mix for all four pieces. Brush the edges of the pastry squares with the egg mixture. Fold each corner over the center of the chicken and seal all the edges firmly. Do this for all four pieces of chicken. Turn the assemblies over and place on a baking sheet (seams down). Refrigerate for later cooking or continue on. Brush the tops with the egg mixture just before putting it into the oven. If the egg mixture saturates into the pastry sheets very long, it will keep it from puffing up, so only brush the egg mixture immediately before baking.
Bake 25-35 minutes until golden brown.