Preheat your oven to 425 F. Prepare your glass containers with cooking spray. Roll out your pie crust and cut strips that are slightly smaller than the height of your container (you'll need room for the container lid to close). Gently press the strips into the sides of your meal prep containers, pressing together strips as needed. (I skipped the bottom because it's so easy for them to get soggy and I wanted more room for filling. But if you love crust you can keep it! You may have to use scraps later for your pie lid.)
Add 1 tbsp oil to a large skillet or dutch oven over medium heat. After 2 minutes, add the chicken breast. Season it with salt, pepper, half of the sage, and half of the thyme. Rotate the chicken breast every few minutes and let it cook completely through, about 10 minutes. Remove it from the pan and set aside.
Melt the butter in the pan over medium heat until it gets bubbly. Add the onion and cook for about 2 minutes, stirring frequently, until tender. Add garlic and cook for 1 minute more.
Add flour, salt, pepper, mustard powder, thyme, and sage, and stir well until all the onions and garlic are coated and it resembles a thick paste. Continue stirring for another 2-3 minutes until the flour is no longer raw.
Add chicken stock a half cup at a time, stirring constantly to prevent lumps, then add milk. Between each addition, stir until the mixture is fully combined. Continue cooking and stirring until the mixture is bubbly and thickened, about 5 minutes.
Stir in chicken and frozen mixed vegetables. Let mixture return to a simmer for 5 minutes, then remove from heat.
Pour the mixture into pie crust-lined containers. Leave some crust visible on the sides. It might not all fit, but the remaining filling is delicious on its own and it's freezer-friendly (or thin it out with water and make it a soup!). Top with the remaining pie crust and, using your fingers or a fork, seal the lid. Cut 3 slits on top. Using a small brush or the back of a spoon, brush the beaten egg on top of the pie shell. Bake for 25-30 minutes.
You can eat this right away OR let it cool a bit before putting the lid on.