Heat oven to 375°F. Spray 15x10-inch pan with cooking spray. Unroll dough in pan; starting at center, press out dough to edge of pan to form crust. Brush with oil; sprinkle with garlic powder and basil. Bake 10 minutes or until golden brown. Cool 15 minutes.
Meanwhile, in small bowl, mix mayonnaise and horseradish sauce until well blended.
Cut focaccia crust into 8 rectangles. Spread mayonnaise mixture on bottom sides of all rectangles. On 4 rectangles, layer cheese, beef, tomatoes and lettuce over mayonnaise mixture. Cover with remaining rectangles, mayonnaise mixture side down. Cut each sandwich in half.