Combine tomatoes, 3 tablespoons olive oil, basil, red onion, garlic, red wine vinegar, and pepper in a bowl; let stand at room temperature for 15 minutes, then marinate in the refrigerator for 45 minutes.

Set oven rack about 6 inches from the heat source and preheat the oven's broiler.

Brush one side of Baguette slices with remaining 3 tablespoons olive oil; place, oiled-sides-up, onto a baking sheet.

Broil until golden brown, about 2 minutes

Spoon tomato mixture onto toasted Baguette slices; drizzle with balsamic vinegar. Serve immediately.