Preheat the oven to 350. Fill a large pot with water and a generous pinch or two of salt, bring to a boil and add the potatoes. Cook until fork tender, 15-20 mins depending on size.
While the potatoes boil, heat a large cast iron skillet over med-high heat, add olive oil. When oil shimmers, add onion, celery and garlic powder. Cook 3-5 mins until soft, stirring often. Add the ground beef. Cook until no pink remains, breaking into small bits. Salt to taste. To the ground beef, add the frozen corn, peas and carrots, stir to combine. Add the broth, Worcestershire and tomato paste. Cook over med-low heat 5 mins to allow the flavors to marry. Salt to taste.
Once the potatoes have reached peak softness, drain the water. Move the potatoes to the bowl of a stand mixer. Begin mixing on low, add butter and mix until incorporated. Next, add the sour cream and grated Parmesan. Mix until light and fluffy and no clumps remain, about 3-4 minutes. Add salt and pepper to taste.
Scoop the mash potatoes to top the cottage pie meat mixture, spread to cover. Top the mash with the sharp cheddar and bake 15-20 mins until pie is set and cheese is melted. Broil on high 3 minutes to brown the cheese slightly. Remove the pie from the oven and allow to cool for 5-10 minutes. Scoop into large bowls and top with Ketchup, enjoy