Put the potatoes and onion in a 4-quart saucepot with enough water to cover them. Heat the potatoes over medium-high heat to a boil. Reduce the heat to low. Cover and cook the potatoes for 20 minutes or until they're fork-tender. Drain the potatoes and onion well in a colander.
Put the potatoes and onion in a 3-quart bowl and beat with an electric mixer at medium speed until almost smooth.
Put the soup, milk, cream and butter in a 4-cup microwavable measuring cup. Microwave on HIGH for 2 1/2 minutes or until hot. Slowly pour the hot soup mixture into the potatoes, beating with an electric mixer at medium speed until the potatoes are smooth. Season to taste.