Peel the garlic clove and finely chop or press it. Place it in a screw-top jar or bowl. Add the finely chopped basil, balsamic vinegar, and olive oil. Seal the jar and shake well until the vinegar and oil are combined. Let stand for 5 minutes.
While the marinade stands, peel the avocado and cut into thin slices. Cut the mozzarella and tomatoes into thin slices. Arrange the avocado, tomatoes, and mozzarella alternately in a circle on a plate. Season with salt and pepper to taste.
Shake the marinade well again. Drizzle it over the salad using a spoon.