Add peeled and cubed sweet potatoes to a large pot and then cover with cold water and cook.
Drain and return to pot with butter, milk, salt, pepper, and garlic powder. Mash until desired consistency.
Place a large skillet over medium heat and add onion and celery. Cook for 3-4 minutes and then add garlic.
Push vegetables to the side and add ground beef. Cook for 7-8 minutes, or until it is cooked through. Mash to get fine crumbles.
Add the rest of the sauce ingredients. Simmer until the sauce begins to thicken and then slowly add in remaining broth.
Add herbs, salt, pepper, and frozen mixed vegetables and continue simmering until they're heated through.
Spray a baking dish with non-stick spray and spread out the ground beef filling. Spread mashed potatoes over the top and bake at 400° for 25-30 minutes. Serve with a sprinkle of parsley and enjoy!
To Store: Keep leftovers in an airtight container in the refrigerator for up to 3-5 days.
To Freeze: Seal in a freezer-safe, airtight container and freeze for up to 3 months.