Prepare a charcoal grill with hot coals. Brush the grilling rack with olive oil.
In a small bowl, whisk together the garlic, mustard, vinegar, 1/4 cup olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper. Set aside.
Place the cucumber, tomato, basil and capers in a large bowl, sprinkle with salt and pepper and toss together. Set aside.
When the grill is ready, brush 1 side of the onion slices and the peppers with olive oil. Place them, olive oil side down, on the grill and cook for 4 minutes. Brush the other side with olive oil, turn them over and continue cooking an additional 4 minutes. Remove the vegetables from the grill and place on a cutting board. Slice the peppers 1/2-inch thick, separate the onion rings and add them both to the cucumber mixture.
Brush the bread slices on both sides with olive oil and toast them on the grill until golden. Add them to the cucumber mixture. Pour the reserved vinaigrette over the vegetables and toss together. Serve warm.