Brown ground beef in Dutch oven over medium-high heat for 10-12 minutes, breaking up as you go. Transfer to paper towel-lined plate and leave 1 tablespoon fat in pot.
Add butter and onions to pot and sauté 5 minutes until softened. Add garlic, Italian seasoning, and tomato paste; cook 1 minute stirring constantly.
Add broth, Worcestershire sauce, heavy cream, potatoes, and cooked beef to pot. Bring to boil over high heat, then reduce to rapid simmer. Cook with lid slightly ajar for 20 minutes, stirring occasionally, until potatoes are fork-tender.
Use potato masher to lightly mash about half the potatoes in the pot to thicken soup.
Whisk cornstarch with 1 tablespoon cold water until smooth. Add cornstarch slurry and frozen vegetables to soup and stir well. Bring to boil, then reduce heat and simmer 10-15 minutes until vegetables are tender and broth thickens.
Season generously with salt and pepper to taste.