Heat oven to 400°F. Cut off ends of each potato; discard ends. Cut potatoes into 3/8-inch-thick slices (about 3 per potato). Place slices on ungreased cookie sheet. Brush slices with oil; sprinkle each with garlic salt.
Bake at 400°F. for 15 to 20 minutes or until tender and golden brown. Cool 20 minutes or until completely cooled.
Meanwhile, in small bowl, combine mayonnaise, wasabi powder, garlic and pepper; mix well.
To serve, place potato slices on serving platter. Top each with about 1/2 teaspoon mayonnaise mixture. Top with roast beef. Garnish each with olive slice.