Cube the stale bread into bite-sized pieces. If the bread is very dry, you can toast it lightly in a pan with a little olive oil until golden brown.
Chop the tomatoes, cucumber, and red onion into bite-sized pieces and add them to a large bowl.
Add the cubed bread to the bowl with the vegetables, and toss everything together.
In a small bowl, whisk together the olive oil, red wine vinegar, salt, and pepper to create the dressing.
Pour the dressing over the bread and vegetable mixture and toss well to combine. Let the salad sit for at least 10 minutes to allow the bread to soak up the flavors.
Garnish with fresh chopped basil and serve immediately. This salad is best served fresh but can be refrigerated for a few hours for more flavor.
Notes
If you prefer a heartier salad, you can add mozzarella or olives to the mix. For a gluten-free option, use gluten-free bread.