In a small bowl, stir together the olives and lemon zest; set aside. In a blender, puree the tomatoes with their juice until smooth, about 30 seconds. Set aside. In a 12-inch skillet over medium, melt the butter. Add the onion and ½ teaspoon salt. Cook, stirring, until softened, about 5 minutes. Stir in the garlic, cumin, coriander, cinnamon and pepper flakes, then cook until fragrant, about 30 seconds. Remove from the heat. Transfer ½ cup of the onion mixture to a large bowl and set aside.