- Brush a baking sheet with olive oil and arrange the baguette slices on the sheet in a single layer. Brush the tops with oil, and season with salt and pepper.
- Bake until crisp and golden, 15 to 18 minutes.
- If not using immediately, let the crostini cool, then store in an airtight container at room temperature until ready to use.
- Core, seed and dice the tomatoes. In a bowl, stir together the tomatoes, half of the basil, the onion, garlic, vinegar, the 1/4 cup olive oil, salt and pepper.
- Slice the mozzarella into 1/4-inch-thick pieces or into smaller pieces so they fit on the crostini.
- To assemble, lay a piece of mozzarella on each crostini and spoon 1 to 2 tablespoons of the tomato salad on top. Garnish each with a pinch of the remaining basil. Serve immediately.