In a medium skillet heat 1 TBS cooking oil over medium high heat. Stir in diced celery and onion and cook for 2-3 minutes until slightly softened. Stir in frozen peas and carrots. Season with 1/4 tsp each of salt, black pepper, garlic powder, and onion powder. Cook for 1-2 minutes until peas and carrots are heated through.Turn off heat and allow to cool slightly while make the meatloaf mixture. In a large bowl mix together ground beef, bread crumbs, milk, egg, ketchup, dried parsley, Worcestershire sauce and 1/4 tsp each of salt, black pepper, garlic powder, and onion powder until combined. Using a wooden spoon mix in the heated vegetables (carrots, peas, onion, and celery). DO NOT OVER MIX AS TO PREVENT SMASHED PEAS AND CARROTS.
Fit a piece of parchment into a loaf pan (9" x 5" x 3). This will aide in removing meatloaf from loaf pan. Place meatloaf mixture into prepared loaf pan.
Bake meatloaf in 350 degree preheated oven for 25 minutes.
While meatloaf is in oven, make mashed potato topping.
Peel and cut 5 medium russet potatoes. Rinse off cubed potatoes and place into a large pot. Fill with water to 1" above the potatoes and season with 1 tsp of salt. Place on burner and bring to a boil over high heat. Allow to simmer until potatoes are fork tender.
Drain potatoes and place potatoes back into pot. Mash potatoes with milk, butter, 1/2 of the shredded cheese, garlic powder, salt & pepper to taste.
Remove meatloaf from oven. Spoon mashed potatoes on top of meatloaf and smooth evenly. Top with additional 4 oz shredded cheese. Top with a sprinkling of dried parsley.
Place meatloaf back into the 350 degree oven for another 25-35 minutes until meatloaf is cooked through and reads 165 degrees using a kitchen thermometer.
Let meatloaf rest in pan for 10 minutes to set up. Remove meatloaf from pan using the edges of the parchment and place onto a serving platter. Pull parchment out from the bottom of meatloaf. Slice and enjoy.