Cook the potatoes in a pot of boiling, salted water until very tender, about 15 minutes.
Meanwhile, bring the cream and butter to simmer in a heavy, small saucepan over medium heat, stirring often. Mix in the green onions. Remove from heat. Cover and let steep while the potatoes cook.
Drain the potatoes thoroughly. Return the potatoes to the same pot and mash. Add the 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.)
Notes
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.