Preheat the oven to 425°F (220°C).
In a skillet, melt the butter over medium heat and sauté the onions until translucent.
Stir in flour and cook for 1-2 minutes to eliminate raw taste.
Gradually whisk in the chicken broth and milk, stirring continuously until thickened.
Stir in the cooked chicken, frozen vegetables, thyme, salt, and pepper.
Roll out the pie crusts and cut into circles to fit a muffin tin.
Press the rounds into the muffin tin, fill with chicken filling, and top with more crust.
Brush with beaten egg for a glossy finish before baking.
Bake for 20-25 minutes until crust is golden brown.
Let cool slightly before serving.
