How to Make It
Bring first 3 ingredients to a boil in a large saucepan; cover, reduce heat, and simmer 30 minutes or until tender. Drain and transfer to a mixing bowl.
Melt 2 tablespoons butter in saucepan; add celery and onion, and saute 3 to 4 minutes or until tender.
Add celery mixture to potatoes; beat at low speed with an electric mixer until potatoes are mashed. Add remaining butter, milk, salt, and pepper; beat at high speed until whipped. Serve immediately.