How to Make It
Remove and discard root ends and dark green tops of leeks. Cut each leek in half lengthwise; rinse thoroughly under cold running water to remove grit and sand. Thinly slice leeks crosswise to equal about 4 cups.
Heat a large skillet over medium-low. Add oil; swirl to coat. Add leeks to pan; cook 25 minutes or until very soft and caramelized, stirring occasionally. Stir in spinach, salt, and pepper; cook 2 minutes, stirring until spinach wilts. Remove pan from heat; cool leek mixture completely.
Combine yogurt, chives, vinegar, and sour cream in a bowl. Stir in spinach mixture. Refrigerate until ready to serve.