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.
In a bowl, whisk together the yogurt, olive oil, lemon juice and mint or dill. Season with salt and pepper. Toss with the spinach and taste for seasoning. Transfer to a serving bowl and sprinkle with walnuts and more herbs. Serve at room temperature with pita bread.