In a 12-inch frying pan, thoroughly cook the ground beef. Drain the fat and set aside for now.

In a large 5-quart pot with 1 tablespoon of butter over medium-high heat, sauté the onion, carrots, and diced celery with the Italian seasoning, oregano, salt, and pepper until onions have softened, 2 to 3 minutes.

Add the beef, potatoes, and chicken broth to the pot. Reduce the heat to low, cover with a lid, and simmer for 10 to 12 minutes, until potatoes are tender.

In a separate small frying pan, melt the remaining 3 tablespoons of butter over medium-high heat and stir in the flour until it starts to bubble, 2 minutes. Then add this mixture to the soup, stirring well to mix.

Stir in the cheese until fully melted.

Add the milk and bring the soup back to a simmer. Then stir in sour cream until well mixed.

Remove from heat and serve hot.