Lightly grease crockpot with butter or nonstick spray
In a mixing bowl, combine cream cheese, sour cream, mayonnaise, garlic, salt, pepper, and optional red pepper flakes until smooth
Fold in drained spinach, mozzarella, and Parmesan; add optional ingredients if desired
Transfer mixture to the crockpot and spread evenly
Cover and cook on low for 1–2 hours, stirring once halfway through
Stir once more before serving and serve warm with dippers