Preheat oven to 350.
Place diced potatoes in large pan filled with water (water should be about 2 inches above the potatoes in the pot). Place pot on stove top over medium heat, bring to a boil, and then reduce to a simmer - cook the potatoes until they're soft (about 25 minutes total).
While the potatoes are simmering, place the ground beef in a large skillet over medium heat and begin to brown.
When beef is about halfway done cooking, add the onions and carrots and continue cooking until meat is no longer pink.
Add zucchini, squash, broth, Worcestershire sauce, tomato paste, Italian seasoning, paprika, Garlic salt and pepper, and sugar to the beef. Stir until well blended.
Cover the skillet and continue to cook beef & veggies over medium heat for 20 minutes.
When the potatoes are soft, drain the water and pour them into a large mixing bowl.
Add butter to the potatoes. Using either a hand mixer or stand mixer, whip the potatoes and butter until the potatoes are fully mashed and the butter is melted. Add heavy cream, sour cream, salt & pepper and mix until ingredients are fully blended. Set aside.
Going back to the beef and veggies - once you've allowed them to cook together for about 20 minutes on the stove top, add frozen peas and corn, cover, and cook for 5 more minutes.
Spoon all of the beef and veggies into a 9x13" pan.
Spread the potatoes evenly on top of the beef & veggies.
Place in oven, uncovered, for 30 minutes.
Remove from oven, sprinkle cheddar cheese on top, and bake in oven for an additional 10 minutes.
You can serve immediately or set aside to cool and store in the fridge or the freezer. This makes great leftovers!