Place a 6-quart or larger pot over medium-high heat and add the Italian sausage. Break it apart with a wooden spoon as it cooks, letting it brown for 5-7 minutes until no pink remains. Remove the cooked sausage to a paper towel-lined plate to drain excess fat.

Reduce heat to medium and melt 2 tablespoons of butter with the olive oil in the same pot. Add your mirepoix (the diced carrot, celery, and onion mixture) and sauté for 5-10 minutes until the vegetables soften and become fragrant. Stir in the minced garlic and cook for just 30 seconds, watching carefully so it doesn't burn.

Add the chopped potatoes, dried parsley, and basil to the pot. Season with salt and pepper, then pour in the chicken stock. Bring everything to a vigorous boil, then reduce heat to low, cover, and simmer for 15-20 minutes. The potatoes should be fork-tender when ready.

While the soup simmers, prepare your cream base. Melt the remaining 2 tablespoons of butter in a separate saucepan over medium heat. Whisk in the flour and stir continuously for about 2 minutes to create a smooth roux. Gradually add the milk while whisking constantly, then continue whisking for 3-5 more minutes until the mixture thickens.

Gently pour the thickened milk mixture into your soup pot, stirring well to combine and avoid splashing hot liquid. Add the shredded cheddar cheese and sour cream, mixing until both melt completely into a creamy consistency. Return the drained sausage to the pot and stir everything together. Taste and adjust seasoning with additional salt and pepper if needed.

Ladle the hot soup into bowls and serve immediately with buttered bread for dipping. The soup thickens as it cools, so add a splash of milk or broth when reheating if needed.