1. Add the leeks and potatoes to the salted boiling water and cook, covered, at a light boil until the potatoes begin to cook apart, or when pressed, the potato offers no resistance to crushing - about 30-40 minutes.
2. Remove soup from heat.
3. Puree 1/3 of the soup in a blender or food processor, return to stove on low heat.
4. Add butter and cheese at the moment of serving (after removal from heat).
5. Serve with bread