Line 2 baking sheets with parchment. On a lightly floured surface, roll pastry pieces into 4 6-inch squares. Place on 1 baking sheet, cover and chill.
Melt butter in a skillet over medium heat. Sauté onion until softened, about 3 minutes. Add peas and carrots; sauté 3 minutes. Sprinkle thyme and flour over vegetables; sauté 1 minute. Raise heat to medium-high. Stir broth and mustard into pan. Add chicken; cook until sauce is thick, 7 to 10 minutes. Transfer to a bowl; let cool. Season with salt. Preheat oven to 375ºF.
Place 1 pastry square on reserved baking sheet. Brush edges with egg. Spoon 1/4 of chicken mixture into center; fold to form a triangle. Seal edges. Repeat with remaining pastry and filling. Brush with remaining egg. Cut 3 slits in top of each. Chill for 15 minutes.
Bake turnovers until golden, about 30 minutes. Let cool 5 minutes and serve.
You can cook and dice up your chicken ahead of time. You only need 1 chicken breast, which yielded 1 1/2 cups for me. You could also use rotisserie chicken. I used a 1/2 tsp dry mustard instead of the dijon mustard. Follow the measurements because if you put in any extra veggies or chicken you will not able to fit it all in the pastry squares. This was fun and yummy and I will definitely make again, but to save time I might put the filling into a casserole dish and just cover with pastry. This would be much faster, you just won't have the cute pockets.
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