Preheat oven to 375°F degrees. Have oven-safe dishes ready to fill.
Cook the onion in the olive oil in a pan until the onions are translucent, about 2-3 min. Add the minced garlic and cook for another minute. Add mushrooms and cook for 2-3 more minutes.
Add chicken, canned soup, broth, peas and carrots and cook on medium heat until everything is combined and nice and creamy. Add salt and pepper to taste and remove from heat.
Pour some of the filling mixture into your cooking vessel like a large ramekin or single portion baking dish. Fill almost to the top. Beat egg and water together for egg wash.
Roll out puff pastry and cut to fit the top of the dish with a little overhang all around, this doesn't have to be perfect. Roll edges or crimp to the top of the dish and make a few small cuts in the middle with a knife to vent.
Brush the prepared egg wash on top and place in the oven for 50-60 minutes until the top is golden brown. *Caution, these are HOT when they come out of the oven, so plan for some cooling time...trust me on this one.