Melt butter in a large pot or Dutch oven over medium heat. Sauté the onion, garlic, and celery for 5 minutes until they start to soften. Add in the turkey and brown while breaking up into smaller pieces. Whisk in flour and cook for about 1 minute. Stir in broth, potatoes, and scallions. Season with salt and pepper.
Cover with a lid and simmer until potatoes are tender (about 15-20 minutes). Stir in heavy cream and simmer for 5 minutes more. Taste and adjust the seasoning with salt if needed.
Ladle into bowls, garnish with croutons, cheese, and scallions.