Heat over to 400°F. Place potatoes in ungreased 15x10x1-inch pan. Bake 30 to 40 minutes or until tender. Cool 10 minutes or until cool enough to handle.
Cut potatoes in half. With melon baller, scoop out insides of potatoes into medium bowl, leaving 1/4-inch lining of potato flesh around edges of shells. Sprinkle shells with 1/2 teaspoon of the salt. To potato flesh in bowl, add remaining 1/2 teaspoon salt and remaining ingredients except onions and radishes; mix well, breaking up potatoes.
Cut very thin slice off bottom of each potato shell so potatoes will stand upright. Fill each potato shell with about 1 tablespoon filling mixture. Top with onions. Cover; refrigerate about 1 hour or until chilled. Garnish with radish slices.