In a large bowl, combine the muffin mix, Parmesan cheese and garlic powder. In another bowl, whisk the eggs, salad dressing and butter; stir into dry ingredients just until moistened. Fold in the spinach, cheddar cheese and onion.
Fill greased miniature muffin cups two-thirds full. Bake at 350° for 12-14 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from pans to wire racks. Serve warm. Refrigerate leftovers.