Preheat oven to 425°F. Line large rimmed baking sheet with foil. Gently toss red onions, 5 tablespoons oil and vinegar in large bowl. Sprinkle with salt and pepper. Arrange onions in single layer on prepared sheet. Bake onions until brown at edges and just tender, about 40 minutes. Cool.
Mix mayonnaise, basil, lemon juice, lemon peel and 2 tablespoons oil in small bowl. (Onions and mayonnaise can be made 2 days ahead. Cover separately and chill.)
Spread mayonnaise over cut sides of bread. Place bottom halves on plates. Top with roast beef, onions and arugula. Cover with top halves of bread.
*Ciabatta is an oval-shaped flat Italian bread available at many bakeries and supermarkets nationwide.