In a medium pot or Dutch oven, heat the butter on medium high heat.
Once melted, add onion and celery, sauté until onions are translucent.
Add in your cooked, diced roast beef.
If using gravy without a thickener, add 2 tablespoons of flour to the pan.
Add in gravy mix.
Add water. Let simmer until desired consistency. To thicken, mix 1 tablespoon of flour with water and add to pot. To thin out, add more water.
Once desired consistency is reached, add frozen veggies. Let simmer for 2 minutes and then remove from heat.
Assemble your pies in your freezer friendly containers of choice (see above blog post for my suggestions). Add the gravy mixture to the container first, then top with mashed potatoes.
If enjoying this right away, bake at 375°F for about 20 minutes, until warm throughout.
If freezing, ensure it is fully cooled before covering and placing in the freezer.