Add onion to frozen spinach in saucepan; cook as directed on package. Drain well, pressing out liquid. In medium bowl, beat eggs. Add cheese, salad dressing, butter and garlic powder; mix well. Stir in spinach mixture and muffin mix. Cover; refrigerate 1 hour or until mixture is easy to handle. Butter baking sheets. With tablespoon, shape spinach mixture into 1" balls;arrange on prepared baking sheets. Cover; refrigerate or freeze until serving time. To serve: preheat oven to 350 degrees. Uncover puffs. Bake refrigerated puffs for 12 to 15 minutes (frozen for 18 to 20 minutes) or until brown. Serve warm with mustard for dipping.