Inspired by Julia Child’s classic recipe for Tian de Courgettes au Riz (zucchini gratin), Virginia Willis's version is comforting yet fresh and clean. Use some of your fast-growing garden zucchini in this gratin, which is similar to the classic and well-loved cheesy squash casseroles, minus the heavy mayonnaise. This recipe, from her column Cooking with Virginia, is big enough to serve ten people with generous-sized servings. And with an active time of just 15 minutes, this will soon become a favorite go-to summer recipe. Simply cook the onions and garlic until fragrant, add flour and stir in the roughly chopped zucchini. Add remaining ingredients, including rice, and pour the mixture into a baking dish. Or, to simplify and cut down on dirty dishes, you can cook the entire recipe in a large ovenproof skillet. Bake this gratin at your next family reunion.
Preheat oven to 425°F. Combine panko and 1/4 cup of the Parmigiano-Reggiano in a small bowl, and set aside.
Heat oil in a large skillet over medium. Add onions, and cook, stirring often, until golden brown, 5 to 7 minutes. Add garlic, and cook, stirring constantly, until fragrant, 45 to 60 seconds. Add flour, and stir to combine. (Mixture will be dry.) Gently stir in zucchini. Add milk, rice, thyme, cayenne, and remaining 1/2 cup Parmigiano-Reggiano, and stir to combine. Spoon mixture into a lightly greased 13- x 9-inch baking dish. Sprinkle with salt and black pepper. (Or, you can start with a large ovenproof skillet and then bake the gratin in it instead of in a baking dish.)
Bake in preheated oven until set and liquid is absorbed, about 40 minutes. Remove gratin from oven. Increase oven temperature to broil with oven rack 6 inches from heat. Sprinkle top of gratin with panko-cheese mixture, and broil until golden brown, about 5 minutes. Serve immediately.