In a large pot over medium-high heat, add the Italian sausage, breaking it apart as it cooks. Sear the sausage for about 5-7 minutes until nicely browned and fully cooked. Remove the sausage from the pot and let it drain on paper towel.

In the same pot, lower the heat to medium and melt 2 tablespoons unsalted butter with olive oil. Add the mirepoix, diced carrots, celery, and yellow onion, sauteing for 5-10 minutes until tender and fragrant. Stir in minced garlic and let it cook for an additional 30 seconds.

Stir in the chopped baby gold potatoes along with the dried parsley and basil. Season with salt and pepper. Pour in the chicken broth, bringing the mixture to a vigorous boil.

Once boiling, reduce the heat to low, cover the pot, and simmer for 15-20 minutes until the potatoes are fork-tender.

While the soup simmers, melt the remaining 2 tablespoons butter in a separate saucepan over medium heat. Whisk in the flour, stirring continuously for about 2 minutes to create a roux. Gradually add the milk, whisking until the mixture thickens, about 5 minutes.

Gently pour the thickened milk mixture into the soup pot, stirring well to combine. Add the extra-sharp cheddar cheese and sour cream, mixing until both are fully melted and creamy.

Return the sauteed sausage to the pot, ensuring everything is well blended, and adjust seasoning with additional salt and pepper if needed.

Ladle the warm soup into bowls, garnishing with fresh herbs if desired. Serve alongside hearty buttered bread for dipping and enjoy while warm.