In soup pot or Dutch oven, heat 3 tbsp (45 mL) butter over medium heat; cook leeks, celery and ¼ tsp (1 mL) pepper, stirring occasionally, until leeks are soft, about 10 minutes. Stir in flour; cook, stirring often, until just beginning to colour, about 10 minutes. Slowly stir in 4 cups (1 L) cold water and 1¼ tsp (6 mL) salt. Bring to boil, stirring. Reduce heat; cover and simmer for 15 minutes.

Meanwhile, in large bowl, beat yogurt with eggs until thoroughly combined. Uncover saucepan; stir in cheese until melted. Gradually beat 1 cup (250 mL) soup into yogurt mixture; stir into soup until hot but not boiling. Ladle into bowls.