Melt butter in a large saucepan over medium heat. Add celery, onion powder and garlic powder. Whisk in flour and stir until light golden brown, about 5 minutes.
Whisk combined milk and cream into the flour mixture. Add salt, thyme, parsley and nutmeg. Simmer, stirring until the mixture is well blended and smooth, 10 to 15 minutes. Whisk in chicken stock; stir in chicken breast and carrots. Bring soup to the boil.
Cook gnocchi in the boiling soup until they float to the top, 2 to 4 minutes. Stir in spinach; cook until liquid is slightly reduced, about 6 minutes.