Preheat the oven to 375 degrees. Grease a muffin tin with butter, oil, or non-stick spray. Thaw and squeeze the spinach dry.
Add the stuffing, turkey, and spinach to a bowl, then stir gently to combine without breaking up the stuffing too much.
Divide the stuffing mixture between all 12 wells of the muffin tin. Leave the stuffing loosely packed in the cups so that the egg mixture can fill in around the stuffing.
In a medium bowl, whisk together the eggs, milk, salt, and pepper. Divide the egg mixture among the 12 wells, starting with 2 Tbsp per muffin and then adding a little bit more until all of the egg mixture has been used. The egg will not fully cover the stuffing, it will puff up as it bakes.
Bake the muffins for 25-30 minutes or until golden brown and crispy on the top. Allow the muffins to cool slightly, then run a knife around the edges to loosen and remove each muffin. Serve warm.