Julia Child's Zucchini and Rice Gratin (Tian de Courgettes au Riz)

Name: Julia Child's Zucchini and Rice Gratin (Tian de Courgettes au Riz)
Prep time: 10 min
Cook time: 1 h
Total time: 1 h 10 min
Cuisine: European, Mediterranean, French
Ingredients: 2 to 2 1/2 lbs zucchini; 1/2 cup uncooked white rice; 1 cup onions, minced; 5 tablespoons olive oil, divided; 2 garlic cloves, minced; 2 tablespoons all-purpose flour; 1/2 cup milk, as needed; 2/3 cup Parmesan cheese, grated and divided; Kosher salt and freshly ground black pepper, to taste