Preheat oven to 350°F. In a bowl, toss bread cubes with 1 Tbsp. olive oil. Spread on a rimmed baking sheet; bake until crisp, about 10 minutes. Return croutons to bowl and toss in garlic.
In a large salad bowl, combine tomatoes, cucumber, bell pepper and red onion. Season with 1/2 tsp. salt and toss to combine. In a small bowl, whisk mustard, vinegar, black pepper and remaining 1/4 tsp. salt. Slowly drizzle in remaining 3 Tbsp. olive oil, whisking until dressing is emulsified.
Add croutons to salad bowl with vegetables and toss to combine. Pour dressing over and toss until all ingredients are well coated. Let stand at least 10 minutes, tossing occasionally. Sprinkle with basil, toss and serve.