Preheat oven to 400°F (200°C). In a small bowl, combine softened butter, minced garlic (2 cloves), 1 tablespoon olive oil, 1 teaspoon salt and 1 teaspoon freshly ground black pepper. Stir until smooth. Arrange baguette slices on a baking sheet in a single layer. Spread about 1 teaspoon of garlic butter on each slice, coating the top edge evenly.

Toast the buttered baguette slices in the preheated oven for 8–10 minutes, until edges are golden and butter is bubbly. For extra crispness, broil on high for the last 1 minute — watch closely so they don't burn. Remove from oven and set aside on a rack to cool slightly.

Basil-caper relish

While the crostini toast, make the relish. Roughly chop 1/2 cup packed fresh basil, drain and roughly chop 2 teaspoons capers, and mince 1/2 clove garlic. In a small bowl, whisk together chopped basil, capers, minced garlic, 2 tablespoons extra-virgin olive oil, 1 teaspoon lemon juice, 1/4 teaspoon red pepper flakes (optional) and 1/4 teaspoon salt until well combined. Taste and adjust salt or lemon as needed. Set aside at room temperature to let flavors meld.

Balsamic reduction & finish

Make the balsamic reduction: in a small saucepan combine 3/4 cup balsamic vinegar and 1 teaspoon sugar or honey. Bring to a gentle simmer over medium heat, then lower heat to maintain a slow simmer. Cook, stirring occasionally, until the volume is reduced by about half and the liquid coats the back of a spoon, about 8–12 minutes. It will thicken more as it cools. Remove from heat and let cool slightly.

When ready to assemble, slice the fresh mozzarella and tomatoes into rounds roughly the same diameter as the crostini slices. Lightly season tomato rounds with the remaining 1/2 teaspoon salt and 1/2 teaspoon black pepper.

Top each crostini with one slice of mozzarella, then a tomato round. Spoon about 1 teaspoon of the basil-caper relish over the tomato. Scatter quartered olives (about 1 tablespoon per crostini) and a torn basil leaf on each.

Drizzle each crostini lightly with extra-virgin olive oil (about 1/12 teaspoon each) and finish with a small drizzle of balsamic reduction (about 1/2 teaspoon per crostini). Serve immediately while crostini are still slightly warm so the mozzarella softens slightly.