In medium serving bowl, add the tomatoes and season with salt and pepper, to taste. Let sit for 10 minutes before adding the beans, tuna, shallots and olives. Just before serving, mix in the bread cubes and combine gently.
In a small bowl, whisk together the mustard, vinegar and salt and pepper, to taste. Drizzle in the oil, whisking to blend. Pour over the salad just before serving. Add the basil and toss. Taste for seasoning before serving.