Prep the ingredients: Wash and dry all produce. Preheat the oven to 400 degrees. Halve the tomatoes. Slice the mozzarella into small pieces.
Roll out the dough: Using your hands, stretch out the dough into a rough ¼-inch thick rectangle (approximately 8̋ x 12̋ ). Place onto a lightly oiled baking sheet and place in the oven for 5-8 minutes, until beginning to brown on the edges. HINT: If you have a rolling pin or wine bottle, you can use it to roll the out dough on a lightly floured surface.
Top the flatbread: Spread the flatbread with 3 Tablespoons pesto and top with the sliced mozzarella and tomatoes. Return to the oven until the cheese begins to brown and bubble, for 6-8 minutes.
Dress the salad: Toss the arugula in a medium bowl with a drizzle of balsamic and olive oil. Season to taste with salt and pepper.
Garnish the flatbread: Tear the basil leaves into small pieces. Roughly chop the walnuts. Sprinkle the walnuts and basil over the heirloom tomato flatbread and top it with the dressed arugula. Cut the flatbread into squares and enjoy!