Remove the tough green outer leaves of the leeks. Cut in two lengthwise and rinse out any dirt. Slice into 5cm lengths.
Place the leeks, potatoes, garlic and water in a pressure cooker. Cook under pressure 20-30 minutes (it doesn't matter if they're a bit overcooked).
When vegetables are cooked, pour the soup into a blender (or use a hand blender). Add water or chicken stock to desired consistency. Add salt and pepper to taste and puree until smooth.