In a large pot over medium-high heat, add the Italian sausage, breaking it apart with a wooden spoon. Cook for 5 to 7 minutes until it is richly browned and cooked through. Drain any excess fat.

Reduce the heat to medium and add unsalted butter to the same pot. Once melted, add the mirepoix (carrots, celery, and yellow onion) and sauté for 5 to 10 minutes until the vegetables soften. Stir in the minced garlic for an additional 30 seconds.

Add the baby gold potatoes, dried parsley, basil, salt, and pepper. Pour in the chicken stock and bring to a boil. Once bubbling, lower heat, cover the pot, and simmer for 15 to 20 minutes until the potatoes are fork-tender.

In a separate saucepan, melt the remaining butter over low heat. Whisk in the flour and cook for about 2 minutes until lightly golden. Gradually add the milk while whisking, cooking for an additional 5 minutes until thickened.

Gently mix the roux into the soup. Add the shredded cheddar cheese and sour cream, allowing them to melt. Return the browned sausage to the mixture and adjust seasoning to taste.

Ladle the soup into warm bowls, garnishing with fresh herbs if desired. Serve with hearty buttered bread.