Add the butter to a large soup pot or Dutch oven over medium heat. Once the butter has melted, add the onion, carrots, and celery. Cook for about 4-5 minutes, stirring occasionally. Add garlic and sauté 20-30 seconds.
Sprinkle the flour and stir for about 2 minutes. Slowly whisk in the chicken broth, stir, then slowly whisk in the milk and stir again until mixture is creamy and combined. Add cauliflower, fresh thyme, and nutmeg. Bring to a simmer and cook for 20 minutes. If the soup is too thick, thin out with water by adding ½ cup at a time until desired consistency is reached.
Add in the prepared gnocchi and spinach. Cook 3-4 minutes until the gnocchi is fork-tender and the spinach has wilted. Season with salt and pepper, and top with grated fontina. Enjoy!