In a medium bowl, combine stuffing mix, cheese, onion, garlic, thyme, and pepper, mixing well.
In a separate medium bowl, combine spinach, broth, and egg whites, mixing well. Stir into the stuffing mixture, cover, and refrigerate for 1 hour or until the mixture is firm.
Preheat the oven to 350ºF and shape the mixture into 24 balls. Place the balls onto an ungreased baking sheet. Bake at 350 degrees for 15 minutes or until they are golden brown. Serve with mustard for dipping, if desired.