1. Peel and slice the leeks.
2. Peel and slice the onions.
3. In a saucepan, melt the butter.
4. Add the sliced leeks and onions.
5. Let them sweat for 10 minutes.
6. Peel the potatoes.
7. Slice them.
8. Add them to the saucepan.
9. Season with salt, pepper, and nutmeg.
10. Add the milk and water.
11. Mix well.
12. Cook over low heat for 30 minutes.
13. Blend everything together.