Scrub and peel 3 to 3 1/2 pounds russet potatoes; cut into 1 1/2-inch chunks. Put them in a 4- to 5-quart pan with 2 1/2 quarts water. Bring to a boil over high heat; reduce heat, cover, and simmer until potatoes mash easily when pressed, 20 to 25 minutes.
When potatoes are done, heat 1 to 1 1/2 cups half-and-half (light cream) or fat-skimmed chicken broth with 2 tablespoons butter or margarine in a microwave-safe container in a microwave oven at full power (100%) just until steaming (don't boil), 1 to 3 minutes. Or warm in a 1- to 1 1/2-quart pan over medium heat.
While cream heats, drain potatoes and mash in pan with a mixer or potato masher until almost smooth. Add cream, a portion at a time, and mix until potatoes have the consistency desired. Stir in salt, pepper, and 1 to 2 tablespoons prepared horseradish to taste. Mound beside Merlot pot roast or scrape into a bowl.