Measure out the sour cream and half and half. Let them all sit out at room temperature. Cook and crumble the ground beef over medium-high heat until cooked through. Drain grease. Remove the ground beef from the pot and set aside.
While the meat cooks, peel the potatoes and cut them into thirds. Add to a stock pot and cover the potatoes with 1 inch of water. Add the salt and boil gently for 10-15 minutes or until very fork tender. Drain, then gently mash with 1 Tablespoon of butter and sour cream. Set aside.
Melt 2 Tablespoons of butter over medium heat. Add the onions and cook until softened, about 5 minutes. Add the garlic and cook for 1 more minute.
Whisk in flour, use a silicone spatula to stir as the flour cooks for 1 full minute to remove the raw flour taste.
Add the chicken broth in splashes, stirring continuously. Use a silicone spatula to loosen any beef remnants from the bottom of the pot, this will add flavor.
Add the Worcestershire sauce, Italian seasoning, mustard powder, and sage. Slowly add the half and half. Bring to a boil, then reduce to a simmer. Fold in the potatoes until combined into the broth.
Add the ground beef and frozen vegetables and allow them to heat through, about 5 minutes. Remove from heat. Gradually sprinkle with shredded cheese and stir until combined. Make sure the base of the soup isn’t too hot or the cheese won’t melt creamy and smooth. The soup will continue to thicken as it sits.