01 - Set oven temperature to 400°F and line a baking sheet with parchment paper.
02 - Warm olive oil in a large skillet over medium heat. Add diced onion and cook until softened, roughly 3 minutes. Stir in minced garlic and continue cooking for 1 minute.
03 - Add chopped spinach to the skillet, stirring until wilted, approximately 2 to 3 minutes. Remove from heat and let cool slightly.
04 - In a mixing bowl, blend the spinach mixture with cream cheese, mozzarella, Parmesan, breadcrumbs, egg, nutmeg, salt, and black pepper until uniformly incorporated.
05 - Using a tablespoon or small scoop, shape the mixture into bite-sized balls and arrange them on the prepared baking sheet.
06 - Place the baking sheet in the oven and bake the bites for 18 to 20 minutes until golden and firm.
07 - Allow the bites to cool slightly before serving.