Preheat oven to 375°F (190°C). Lightly grease a mini muffin tin or line with paper liners.
In a medium skillet, sauté the chopped onion in melted butter over medium heat until soft and translucent, about 5 minutes. Set aside to cool slightly.
In a large mixing bowl, combine drained spinach, stuffing mix, beaten eggs, Parmesan cheese, shredded Cheddar cheese, and the sautéed onions. Mix well until all ingredients are evenly incorporated.
Spoon the mixture into the prepared mini muffin tin, pressing down lightly to pack each cup full.
Bake for 18-20 minutes, or until the bites are set and golden brown on top.
Allow to cool for a few minutes before removing from the tin. Serve warm or at room temperature as a savory appetizer or snack.