Heat up oil in a pan. Sauté onions and celery. Add the ground beef. Cook and stir until browned.
Discard as much grease as you can. Stir in the garlic powder, salt, and pepper seasonings.
Add the frozen peas and carrots. Stir and cook for 5 to 8 minutes.
Sprinkle flour. Stir the whole thing together a couple of minutes then turn the heat off. (I learned this step from AllRecipes: https://bit.ly/3lnLjGg Let it cool for a little bit.
Preheat the oven to 350°, Transfer the meat filling into a pan.
Carefully spread the mashed potatoes on top.
Bake it in the oven for 20-25 minutes.