On the stovetop, cook the ground beef and onion on medium heat. As the beef and onion cook, add the thyme, garlic powder, 1 tsp salt, and black pepper.
Once the beef mixture is cooked through, transfer it to the crock pot.
Add the frozen vegetables, tomato paste, tomato juice, and ½ tsp salt. Stir to combine.
Evenly spread the mashed potatoes on top of the beef mixture.