Preheat the oven to 400 degrees. Coat a shallow baking sheet pan with cooking spray.
Combine the basil and walnuts in the food processor and pulse; with the machine running, add the oil and water in a slow steady stream. Add the parmesan, pulse to combine and set aside.
Lay the zucchini out in a single layer on the baking sheet and season with the salt. Sprinkle ¼ cup of the shredded mozzarella on the zucchini. Top with a tomato slice. Spread each tomato with about 1 teaspoon pesto and top with the remaining mozzarella.
Bake until the cheese is melted and golden and the zucchini is fork tender, about 22 minutes.