How to Make It
In a large saucepan, melt the butter in the olive oil. Add the onion, garlic and a generous pinch each of salt and pepper and cook over moderate heat, stirring, until softened and caramelized, 20 minutes.
Add the spinach to the saucepan in large handfuls, allowing each to wilt slightly before adding more. Cook over moderate heat, stirring occasionally, until the spinach is wilted and very tender, about 20 minutes. Scrape into a medium bowl, season with salt and pepper and let cool completely.
Add the yogurt to the spinach and mix well. Season with salt and pepper and refrigerate until slightly chilled, 15 minutes. Serve with pita chips and za’atar.