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 did not include a bottom because I wanted more room for filling (and nobody likes a soggy bottom), 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.
Add your butter to the pan over medium heat and allow it to melt and get 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. This should resemble 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 bubbly 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. 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 whisked 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.