Make croutons with the bread cubes, olive oil, garlic, and salt following the directions on pp. 30-31, toasting them over lowest heat until all the moisture is drawn out and they are very crisp, Let cool. Meanwhile, place tomatoes and onion in a large bowl. In a small bowl whisk together the salt, pepper, mustard, and vinegar until the salt is dissolved. Whisk in the oils until well blended and pour over the tomatoes. Toss well and let sit for 1 hour at room temperature. Just before serving toss the dressed tomatoes and onion with the croutons, greens, cheese, and nuts. Serves 4 to 6