Heat a grill to medium heat. Brush the grilling rack with olive oil.
In a small bowl, whisk together the garlic, mustard, vinegar, 1/4 cup olive oil, salt and black pepper to taste. Set aside.
Place the cucumber, tomatoes, basil and capers in a large bowl. Season with salt and pepper and toss.
Brush 1 side of the onion slices and peppers with olive oil. Place them oil side down on the grill and cook for 4 minutes. Brush the opposite side with olive oil and flip. 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 on both sides with olive oil. Place them on the grill until golden. Pour the vinaigrette over the vegetables and toss together. Serve with toasted bread.