Shepherd's Pie Soup

Ingredients

• 4 large russet potatoes, peeled and cut into thirds
• 3/4 teaspoon salt, plus more, to taste
• water, to taste
• 3 tablespoons butter, divided
• 3/4 cup sour cream, room temperature
• 1 pound ground beef, 85% lean
• 1 large yellow onion, chopped
• 3 cloves garlic, minced
• 1/4 cup flour
• 4 cups chicken broth
• 3/4 teaspoon Worcestershire sauce
• 2 teaspoons Italian seasoning
• 1/2 teaspoon mustard powder
• 1/4 teaspoon ground sage
• 2 cups half-and-half, room temperature
• 1 1/2 cups frozen mixed vegetables
• 2 cups cheddar cheese, room temperature, shredded
• black pepper, to taste