Heat a Dutch oven over medium-high heat and brown the sausage, breaking up any large clumps. Remove from the pot and drain the meat on paper towels.
Add the onion, celery, and garlic to the pot and cook until the vegetables are tender.
Add the potatoes, chicken stock, and dried sage, and bring to a boil, then reduce to a simmer.
Cook until the potatoes are very tender and fall apart when pierced with a fork.
Transfer the soup in batches to the pitcher of a blender and carefully puree until smooth.
Strain the soup through a fine-mesh sieve into a large soup bowl..
Whisk in the heavy cream and sausage and serve immediately with the cheese, sage, and scallions on the side for garnish.