Make the balsamic reduction by adding the balsamic vinegar to a small saucepan over medium-high heat. Bring the balsamic to a boil, whisking constantly. Reduce heat and simmer for 10-15 minutes or until balsamic has reduced by half and is thick enough to coat the back of a spoon. Make sure you keep an eye on it -- it burns quickly.
Slather each slice of bread with a drizzle of olive oil and toast.
Evenly divide the avocado between the pieces of toast and use the back of a fork to smash the avocado. Layer the sliced tomatoes on top, sprinkle with fresh basil, and drizzle with the balsamic reduction.