Preheat oven to 425°. On a large sheet pan, toss green beans with oil and salt. Bake, tossing after 7 minutes, until beans are crisp-tender, 10 to 15 minutes.
Toss green beans again; sprinkle with garlic and ¼ teaspoon pepper. Top with Parmesan and mozzarella.
Heat broiler. Broil beans, watching closely, until cheese is melted and golden brown, 2 to 3 minutes. Season with more pepper and serve.
