Make vinaigrette: In a medium bowl, with a whisk, beat together 1/2 tablespoon balsamic vinegar, mustard, salt, pepper. Add 1/2 tablespoon olive oil and beat until emulsified. Add the other 1/2 tablespoon of vinegar and 1 more tablespoon of olive oil and beat until emulsified. Top the mixture with the remaining 1 1/2 tablespoons of olive oil. Add the garlic clove. Refrigerate until ready to use.
On 1 slice each of bread layer sliced avocado and tomato. Drizzle the vinaigrette over sandwiches. Top with a sprig of cilantro.