Champ is traditionally served with a well in the middle that has a dab of butter melting in it. The potatoes are usually eaten from "outside" to "inside," dipping each bite into the butter. From the Tinakilly Country House & Restaurant in Rathnew, Ireland.
Cook potatoes in pot of boiling salted water until very tender, about 15 minutes.
Meanwhile, bring cream and butter to simmer in heavy small saucepan over medium heat, stirring often.
Mix in green onions. Remove from heat. Cover and let steep while potatoes cook.
Drain potatoes thoroughly. Return potatoes to same pot and mash. Add cream mixture and stir until blended. Season to taste with salt and pepper. (Can be prepared 2 hours ahead. Cover; let stand at room temperature. Rewarm over low heat, stirring often.)