Cook unpeeled potatoes in boiling water until tender, 20 to 25 minutes.
Drain and cool enough to handle; peel if desired.
In same saucepan, combine butter and onion.
Cook until onion is tender.
Blend in flour, salt, dill and pepper.
Stir in milk, mixing well.
Cook until mixture boils, stirring constantly.
Boil 1 minute.
Add potatoes and peas.