Preparation: Shred the cheddar cheese from a block, measure out the sour cream and half and half, and set them aside to come to room temperature.
Cook the Beef: In a large pot, brown and crumble the ground beef over medium-high heat until fully cooked. Drain any excess fat, remove the beef from the pot, and set it aside.
Boil and Mash Potatoes: While the beef cooks, place the cubed potatoes in a stock pot, covering them with about an inch of water. Add ½ teaspoon of salt, then bring to a gentle boil. Cook for 10-15 minutes until the potatoes are very tender. Drain and mash them with 1 tablespoon of butter and the sour cream. Set the mashed potatoes aside.
Prepare the Broth: In the same pot used for the beef, melt the remaining 2 tablespoons of butter over medium heat. Add the diced onion and sauté until softened, about 5 minutes. Add the minced garlic and cook for an additional minute.
Thicken the Soup: Sprinkle in the flour and stir continuously for 1 minute, allowing the flour to cook. Gradually add the chicken broth, stirring well to incorporate. Use a spatula to scrape any browned bits from the pot’s bottom to enhance the flavor.
Season: Add Worcestershire sauce, Italian seasoning, mustard powder, and ground sage. Slowly pour in the half and half, stirring well. Bring the mixture to a gentle boil, then lower the heat to simmer.
Combine Potatoes: Add the mashed potatoes to the broth, stirring to blend. For an extra creamy consistency, use an immersion blender to puree until smooth, or blend in batches using a countertop blender.
Add Beef and Vegetables: Stir in the reserved ground beef and frozen vegetables, allowing them to heat through for about 5 minutes.
Incorporate Cheese: Remove the soup from the heat. Gradually add the shredded cheddar cheese, stirring gently to melt. Ensure the soup base is not overly hot, as high temperatures can cause the cheese to separate.
Serve: Taste and season with additional salt and pepper if needed. Ladle the soup into bowls and enjoy!