Melt butter in a large pot over medium heat. Cook drained leeks and celery for 6 minutes until soft, then stir in flour for 1 minute.
Gradually stir in broth, scraping the bottom smooth. Add chicken, thyme, salt and pepper and bring to a gentle simmer.
Cook for 15 to 18 minutes until the chicken pieces reach 165°F. Stir in kale and simmer for 3 minutes.
Add gnocchi and cook for 3 to 5 minutes, following the package guidance, until tender and floating.
Stir in cream and heat gently for 2 minutes without a hard boil. Serve while the gnocchi are still light.