Preheat your oven to 350°F (175°C). Place one pie crust in a 9-inch dish and set aside.
In a skillet, melt butter over medium heat. Add chopped onion and cook until translucent, about 3 minutes.
Stir in flour and cook, whisking constantly, for 1–2 minutes until golden.
Gradually whisk in chicken broth and milk. Simmer until sauce thickens, about 3–4 minutes. Season with salt and pepper.
Stir in the cooked, diced chicken breast and frozen mixed vegetables. Cook for another 2 minutes until everything is heated through.
Pour the chicken mixture into the prepared pie crust. Top with the second crust, seal edges, and cut vents to release steam.
Bake for 25–30 minutes, or until the crust is golden brown and the filling is bubbling.
Let the pie rest for 5 minutes before slicing.