Prepare grill to medium-high heat. Drizzle the onions, peppers and bread with olive oil and grill until charred. When done, remove and set aside.
Chop onions, peppers and bread into desired size, about 1 in pieces. Place in bowl.
Add the chopped tomatoes, cucumbers, basil and capers to the grilled vegetable mixture.
To make the vinaigrette, whisk together the garlic, mustard, vinegar and olive oil.Toss dressing over salad. Season with salt and pepper.