Preheat your oven to 375°F (190°C). In a large skillet over medium-high heat, cook the ground beef and diced onion, breaking up the meat with a spoon, until the beef is no longer pink and the onion is softened, about 7-8 minutes. Drain any excess fat.

To the skillet with the beef, add the garlic powder, salt, pepper, and dried thyme. Stir to combine. Reduce heat to medium. Add the cream of mushroom soup, milk, sour cream, and Worcestershire sauce. Stir until the sauce is smooth and well combined with the beef.

Stir the frozen mixed vegetables into the beef and sauce mixture. Cook for another 2 minutes, just to thaw the vegetables. Pour the entire mixture into a 9x9 inch or similar 2-quart baking dish, spreading it into an even layer.

In a medium bowl, mix the prepared mashed potatoes with 1 cup of the shredded cheddar cheese until well combined.

Dollop the cheesy mashed potatoes over the beef layer. Use a spoon or spatula to gently spread them into a smooth, even layer, sealing it to the edges of the dish. Sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top.

Bake in the preheated oven for 25-30 minutes, or until the cheese on top is fully melted, the edges are bubbly, and the top is a light golden brown.

Remove the bake from the oven and let it rest for 5-10 minutes. This allows the layers to set for cleaner serving. Garnish with chopped parsley if desired, then scoop onto plates.