Peel and chop potatoes into small chunks and place in a large pot of water. Bring water to a boil and cook for 15-20 minutes, until potatoes are tender.
Drain water from the pot and mash the potatoes with the butter, milk, salt, pepper, and garlic powder. Cover to keep warm, and set aside.
For the meat layer
Preheat oven to 375 degrees and grease a large casserole dish.
In a large skillet over medium heat, cook ground beef and onion. Break up the meat as it's cooking. Cook until browned and onions are translucent. Drain off excess fat.
Add minced garlic, salt, pepper, and oregano and cook for 1 minute, until fragrant.
Sprinkle flour over the meat and onion mixture and cook for 1-2 minutes.
Stir in beef broth, tomato paste, and Worcestershire sauce and heat to a boil.
Reduce heat to medium low and stir in the frozen peas and carrots and corn. Cook for 5-10 minutes until heated through and thickened.
Pour meat mixture into the prepared baking dish and spread evenly in a thick layer across the bottom.
Spread mashed potatoes evenly over the top of the meat layer.
Bake uncovered for 30 minutes, until hot and bubbly. Remove from oven and let stand for 10 minutes prior to serving.
Scoop shepherd's pie into bowls, garnish with fresh parsley, and enjoy!