Peel potatoes, cut into chunks, and boil in salted water until tender, about 15-20 minutes.
Drain potatoes and mash with sour cream and 1 tablespoon butter; season with 1/4 teaspoon salt and set aside.
In a large pot, melt 2 tablespoons butter over medium heat.
Add diced onion and cook until translucent, about 5 minutes.
Add minced garlic and cook for 1 minute more.
Add ground beef to the pot, cook until browned and no longer pink; drain excess fat if needed.
Stir in flour and cook for 2 minutes to create a roux.
Gradually whisk in chicken broth and half and half, stirring constantly until smooth and slightly thickened.
Add Worcestershire sauce, Italian seasoning, mustard powder, ground sage, remaining salt, and pepper to taste.
Simmer for 10 minutes.
Add mixed frozen vegetables and cook for another 5 minutes until tender.
Stir in half of the shredded cheddar cheese until melted.
Ladle soup into bowls and top each serving with a dollop of mashed potatoes.
Sprinkle remaining cheddar cheese on top as garnish.
Serve hot and enjoy your comforting Shepherd’s Pie Soup.