How to Make It
Whisk together the vinegar, honey, and Dijon in a bowl. Slowly whisk in oil until combined. Season with salt and pepper to taste. Set aside.
Slice tomatoes into ¼-inch-thick slices. Season with salt and pepper to taste; set aside. Using a mandolin, thinly slice the cucumber and red onion.
To build the stack, start with the sourdough and top with 1 tablespoon balsamic of the vinaigrette. Top bread with 2-3 slices of tomato and 1 tablespoon vinaigrette. Finish the stack with 2 slices of mozzarella, slices of cucumber, slices of red onion, and additional vinaigrette to taste. Repeat process to create as many layers as desired. Garnish with basil and parsley.