Mix onion, celery, garlic and water together in a bowl. Cover with plastic wrap and microwave for 5 minutes (stir halfway through) until onions are translucent.
Place chicken in the bottom of a slow cooker and sprinkle with seasoned salt. Add the onion mixture, bell peppers, and mushrooms.
Mix the soup and chicken broth together and then pour in the slow cooker. Cover and cook on low for 4 hours.
Mix the cornstarch and water together in a small dish and add to the slow cooker along with the cream cheese and peas. Mix in and cook for 30 minutes, uncovered.
Meanwhile, make the puff pastry. Grease a baking sheet or line with parchment paper. Unfold the pastry sheets and cut each into 4 squares. For each square, cut a thin strip almost all the way through on each side. Wet the dough with your fingers and then pull each thin strip onto the adjacent edge and lightly press down. Continue with each square.
Mix the egg and water together and then lightly brush the square with it.
Bake for 12-15 minutes at 400 degrees until puffed and golden.
Shred the chicken (I just shred it in the slow cooker but you can remove it and add it back). Spoon the chicken mixture over each puff pastry and serve.
Slow Cooker Chicken a la King https://www.garnishandglaze.com/slow-cooker-chicken-a-la-king-puff-pastry-recipe/