Preparation

• Heat oven to 425 degrees. Spread the bread cubes on a rimmed baking sheet and toss with 2 tablespoons oil. Bake until they are golden brown at the edges, about 7 to 15 minutes. Let cool on a wire rack.
  
  Cut tomatoes into bite-size pieces and transfer to a large bowl. Add onions, garlic, 1 tablespoon vinegar, 1/4 teaspoon salt. Toss to coat and set aside.
  
  In a medium bowl, combine remaining 1 tablespoon vinegar, the mustard, 1/4 teaspoon salt and some black pepper to taste. While whisking constantly, slowly drizzle in the remaining 4 tablespoons olive oil until the mixture is thickened. Stir in basil.
  Add bread cubes to the tomatoes and toss well. Let sit for at least 30 minutes before serving. Toss with a little more olive oil, vinegar and salt if needed just before serving.