Place cubed focaccia in a baking tray then lightly drizzle it with olive oil. Season with garlic salt and freshly ground black pepper then place in a 190C preheated oven and bake for 12 minutes or until golden brown in colour. Remove from oven and let it cool.
Combine focaccia cubes, tomatoes, onion, mozzarella cheese and basil in a salad bowl then toss to combine.