Preheat the oven to 375°F (190°C).
Toss ciabatta pieces with ¼ cup olive oil, salt, and pepper. Spread on a baking sheet and bake until golden and crispy, about 10–15 minutes. Let cool.
In a small bowl, whisk together red wine vinegar, ½ cup olive oil, garlic, Dijon mustard, salt, and pepper until well combined.
Halve tomatoes, slice onion, dice cucumbers, and chop basil.
In a large bowl, combine toasted bread, veggies, basil, and mozzarella. Drizzle with vinaigrette and toss gently to coat.
Let the salad sit for 30 minutes so the bread can soak up the dressing and flavors blend.
Taste and adjust seasoning. Drizzle with extra olive oil before serving.