Inspired by Julia Child's classic recipe for Tian de Courgettes au Riz (zucchini gratin), my version is comforting yet fresh and clean--it's not overloaded with heavy cream or cheese.
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.