In a pot melt butter over medium heat. Add leeks Celery and ½ tsp (2 mL) salt and ¼ tsp (1 mL) pepper and cook Stirring often for 5 min or until leeks are tender.
Add potatoes, Broth and 1 cup (250 mL) water; cover and bring to a boil over high heat. Reduce heat to medium-low and boil gently For 15 min or until potatoes are soft. Remove from heat.
Remove to a bowl half of the soup then with an immersion blender purée about the rest until smooth
Return the extracted soup to the pot
Stir in milk; heat over medium heat stirring often just until steaming (do not let boil). Stir in lemon juice and season to taste with salt and pepper.