Preheat oven to 400°F (200°C). Spray a medium-sized baking dish with cooking spray and set aside (I used a deep 5-inch by 9-inch dish, but a 7x11-inch dish or anything similar in size will work well).
Slice zucchini and squash into thin rounds (I like to use a mandolin to make it fast and uniform, but it’s not necessary).
In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add garlic and cook (stirring constantly) for about 1 minute. Add zucchini and squash to the skillet. Stir and cook until vegetables are tender (about 5-6 minutes). Remove from heat.
In a small saucepan, add remaining 1 tablespoon of oil and diced onion. Cook onion over medium heat until translucent (about 2 minutes), and then add the tomatoes (with their juice) and the basil. Simmer tomatoes about 5 minutes. Stir in parsley, salt, and pepper to taste.
Spread half of the zucchini/squash mixture into the bottom of the prepared baking dish. Cover with half of the tomato sauce from the saucepan, and sprinkle with 1 tablespoon of grated Parmesan.
Add another layer with the remaining zucchini, top with the remaining sauce, and finish with the last 2 tablespoons of Parmesan.
Bake for 15 minutes, or until cheese is melted and the top becomes golden brown.
Allow dish to sit for about 10 minutes before serving.
Notes
Recipe inspired by Essentials of Classic Italian Cooking.