Preheat grill or broiler to high. Combine tomatoes, oil, 1 tablespoon salt, and pepper to taste in a large bowl.
Toast cut side of baguette halves on grill or under broiler 1 to 2 minutes. While still hot, lightly rub cut surface with garlic. Spoon tomato mixture over bread with a slotted spoon, then add onion. Tear basil into pieces, and scatter on top. Slice each baguette half into 3- or 4-inch-long pieces (about 12 slices per baguette).