Heat 1 tablespoon of extra-virgin olive oil in a skillet over medium heat. Sauté the chopped onion until tender, around 5 minutes. Add minced garlic and cook until fragrant, about 30 seconds. Stir in quartered tomatoes, salt, pepper, sugar, and basil sprigs, then increase the heat and let simmer until thickened, about 15-25 minutes. Remove basil sprigs and adjust seasoning as needed.

Preheat the oven to 450°F. Cut zucchini lengthwise and season with salt, pepper, and 2 tablespoons of olive oil; toss to coat. Arrange zucchini slices on baking sheets and sprinkle with red pepper flakes. Roast for 12 minutes or until lightly browned.

Reduce the oven temperature to 375°F and lightly oil a 2-quart baking dish. Spread a thin layer of tomato sauce at the bottom, then layer one-third of the roasted zucchini followed by another layer of sauce and 1/4 cup Parmesan. Repeat two more times, finishing with a final layer of sauce and Parmesan on top. Drizzle with any remaining olive oil.

Place the casserole in the oven and bake for 30-35 minutes until bubbling and golden brown. Allow it to sit for 5-10 minutes before serving to set the layers.