Heat a large pot over medium-high heat. Place the Italian sausage into the pot and allow it to sear for a minute before breaking it apart with a wooden spoon. Cook until the sausage is deeply browned, then transfer it to a plate lined with paper towels, leaving about 1 tablespoon of grease in the pot. Add 1 tablespoon of olive oil if the pot lacks sufficient grease.

In the same pot, melt 1 tablespoon butter. Add the diced carrots, celery, and onion (mirepoix) and sauté for 5–7 minutes (or 7–10 minutes if the veggies are larger). Stir in the minced garlic and let it sauté for about 30 seconds more.

Stir in the chopped baby gold potatoes, dried basil, dried parsley, salt, and pepper. Pour in the chicken stock, bring the mixture to a boil, then reduce heat to medium-low, cover the pot, and cook for 15–20 minutes or until the potatoes are fork-tender.

While the potatoes are cooking, melt 4 tablespoons of butter in a separate medium pot over medium heat. Stir in the flour and cook, whisking constantly, for about 1 minute. Gradually add the milk, whisking continuously to achieve a smooth consistency. Cook, continuing to whisk, until the mixture thickens and begins to gently boil. Remove from heat and stir in the heavy cream.

Once the potatoes are tender, stir the milk and cream mixture into the main soup pot. Turn off the heat and add the shredded extra-sharp Cheddar cheese a handful at a time, stirring gently until fully melted. Next, mix in the sour cream, then add the cooked sausage back into the soup.

Give the soup a taste and fine-tune the seasoning with additional salt and pepper, if necessary.

Serve the soup warm, optionally with hearty buttered bread on the side.