How to Make It
Heat the oil in a large skillet over medium-high heat. Add the onions, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring often, until golden brown, 12 to 15 minutes. Transfer to a medium bowl and let cool.
Add the sour cream, spinach, vinegar, ¾ teaspoon salt, and ¼ teaspoon pepper to the onions and mix to combine. Serve with the vegetables.