Cut the bread into approximately 1" cubes and spread on a baking sheet. Drizzle the bread with olive oil and a little salt.
Bake the bread until the cubes are just crunchy but do not squish or bend. They should not be as hard as croutons. The time this takes will depend on how dry/dense your bread already is (for me it typically takes 20-30 minutes). Set aside to cool to room temperature.
While the bread is baking, prepare the mix-ins. Cut the cucumber and tomatoes into 1/2" chunks and place in a large bowl.
Thinly slice the red onion and place in the large bowl with the cucumber and tomatoes. Add the kalamata olives (you can use either whole or sliced).
Cut the feta cheese into 1/2" cubes or use crumbles and add to the vegetables.
In a small bowl, prepare the dressing by combining all of the ingredients. Whisk together and salt to taste.
When the bread is done and cooled to room temperature, add the bread to the vegetables bowl and toss to combine.
Pour the dressing over the salad and toss to coat. Let the dressing sit on the salad ideally for 30 minutes in the refrigerator before serving so that it starts to soak into the bread. It will not make the bread soggy.
Notes
Make it vegan by skipping the Greek yogurt.Ideally you want to use a denser bread for this salad, something more rustic with less air. The weight of the bread will vary based on exactly which one you use, but you want to aim for 6 cups in volume regardless.