Discard the spinach stems. Wash and drain the leaves, then chop coarsely. Melt the butter in a large saute pan. Add the onion and a generous pinch of cinnamon, if using, and cook over medium heat until translucent, about 5 minutes. Add the garlic and saute until fragrant. Add the spinach and cook until it wilts, stirring often. Drain well and let cool.