This is a simple layered casserole with three elements: roasted zucchini, a really good homemade tomato sauce and Parmesan. Roasting, rather than frying the zucchini, allows you to cut down on olive oil and time.
Add garlic. Cook, stirring, until fragrant, 30 seconds to a minute, and add tomatoes, salt, pepper, sugar and basil sprigs. Increase heat to medium-high. When tomatoes are bubbling briskly, stir and reduce heat to medium. Cook, stirring often, until tomatoes have cooked down and are beginning to stick to pan, 15 to 25 minutes, depending on consistency. Remove basil sprigs; taste and adjust seasoning.
04Heat oven to 450 degrees. Line 2 sheet pans with parchment. Trim ends off zucchini and cut in half crosswise, then into lengthwise slices, about 1/4 to 1/3 inch thick. Season on both sides with salt and pepper and toss with 2 tablespoons olive oil. Arrange zucchini slices on baking sheets in one layer and sprinkle with red pepper flakes. Roast for 12 minutes, until lightly browned and easily pierced with a knife. Remove from oven and reduce heat to 375 degrees.
05If using a food mill, put sauce through medium blade. If not, pulse sauce in a food processor fitted with steel blade until just coarsely puréed. Stir in chopped basil.
06To assemble the dish, oil a 2-quart gratin with olive oil. Spread 1/4 cup tomato sauce over bottom of dish. Arrange a third of the zucchini in an even layer over tomato sauce. Spoon a third of remaining sauce over zucchini and sprinkle with 1/4 cup Parmesan. Repeat with 2 more layers, ending with 1/4 cup Parmesan. Drizzle on remaining tablespoon olive oil. Bake 30 to 35 minutes, until bubbling and browned on the top and edges. Remove from heat and allow to sit for 5 to 10 minutes before serving.