Pre-heat the oven to 400°F.
Spray a deep pie dish or casserole dish with non-stick cooking spray.
Run pierogies under cool water to thaw.
In a medium sauté pan, over medium-high heat, heat the butter.
Add the onions; cook for about five minutes, stirring, until onions are translucent.
Add ground beef and continue to cook until the beef is throughly cooked.
Add frozen vegetables, beef gravy, Worcestershire sauce and garlic powder; cook for another 5 minutes.
Season with salt and pepper, to taste.
Allow mixture to cool slightly.
Add the ground beef mixture to the prepared pie or casserole dish and top with pierogies.
Spray the top with non-stick cooking spray and place in the pre-heated oven.
Bake for 25 to 30 minutes or until the pierogies are golden brown.
