Panzanella is a classic Tuscan bread and tomato salad, characteristically enlivened with olives and fresh marjoram. This dish is all about the tomatoes and bread, so don't bother to make it unless you have fabulous, summer-ripe tomatoes. The bread needs to be from a coarsely textured country-style loaf, or it'll get soggy too quickly. Day-old bread is fine to use; drying out the bread cubes as suggested by lightly toasting them allows the salad to be prepared slightly in advance of serving. Adding capers and cheese is non-traditional - but this salad lends itself to improvisation, so use your imagination!
Mash the anchovy fillets, garlic, and salt in a small bowl with a fork (or use a mortar and pestle) until it's a smooth paste. Transfer the paste to a small jar and add the vinegar, oil, and freshly ground pepper to taste. Shake vigorously to blend, then set aside at room temperature while you prepare the salad. The dressing will keep, refrigerated, up to 2 weeks.
Place the bread cubes, tomatoes, olives, capers, herbs, and mozzarella in a large bowl. Add half of the dressing and toss to coat; taste and add more dressing as needed. Let the salad sit at room temperature for 10 to 15 minutes so that the bread can absorb the flavors, then serve.