Build the Soup:
Slowly add the chicken broth in small splashes, whisking or stirring constantly to prevent lumps and to deglaze any flavorful bits from the bottom of the pot. Once the broth is fully incorporated and smooth, stir in the Worcestershire sauce, Italian seasoning, mustard powder, and ground sage. Gradually pour in the half and half while stirring to combine. Increase heat to bring the mixture to a gentle boil, then reduce to a simmer.
Blend in the Potatoes:
Carefully add the prepared mashed potatoes into the simmering broth. Stir well to fully incorporate the potatoes, which will begin to thicken the soup and give it that creamy, shepherd’s pie consistency. For a smoother texture, use an immersion blender to puree the mixture directly in the pot until silky. If you don’t have one, carefully transfer the soup to a blender in batches, blend until smooth, and return to the pot.
Finish the Soup:
Add the cooked ground beef and frozen mixed vegetables directly into the soup base. Stir to combine and let the mixture heat through for about 5 minutes, or until the vegetables are tender and the beef is warmed. Remove from heat.
Cheese and Seasoning:
Let the soup cool just slightly before gradually adding the shredded cheddar cheese. Stir in handfuls at a time until fully melted and incorporated. Avoid adding the cheese when the soup is boiling hot, as this can cause it to separate or become grainy. Once the cheese has melted smoothly, taste the soup and adjust the seasoning with additional salt and pepper if needed.
Serve:
Ladle the soup into bowls and enjoy immediately. Garnish with extra shredded cheese or chopped parsley, if desired. This soup thickens as it sits and reheats well the next day. Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat gently on the stovetop or in the microwave, adding a splash of broth or milk to loosen the consistency if needed.
