Prepare a grill or preheat the broiler. Brush the bread slices lightly on both sides with olive oil. Grill or place the breads on a pan, 6 inches under the broiler, until lightly browned, turning once, 3 to 4 minutes total. Remove from the oven & let cool. The bread slices can be toasted several hours in advance.
To make the topping:
In a medium bowl, stir together the tomatoes, mozzarella, basil, oil, vinegar, garlic, salt, & generous grinding of pepper. Cover & let stand at room temperature for 30 minutes. Adjust the seasoning if necessary. Spoon some of the topping onto each slice of bread, or serve the topping in a bowl accompanied by the grilled bread slices, letting guests top their bruschetta to tastes.