Mix together tomatoes, onions, olives, basil, capers in a small bowl. Whisk together the olive oil, balsamic vinegar, garlic, and black pepper. Pour over the tomato-onion mixture, and toss. Set aside.
Brush the bread with olive oil and grill slices directly over medium heat, turning once, until toasted, 2 to 3 minutes total. Divide the mascarpone evenly among bread slices, spreading it with a knife. Spoon the tomato onion mixture over the mascarpone cheese, dividing evenly and serve at room temperature.