Preheat oven to 425°F.
Toss 1 pounds green beans, trimmed on a sheet pan with 2 tablespoons olive oil, 0.5 teaspoons kosher salt, and 0.3 teaspoons black pepper.
Spread beans in a single layer, not overlapping.
Roast until beans are tender with some browned, blistered spots.
Remove from oven, squeeze 0.5 lemon, zested and juiced juice over the beans and sprinkle with the zest.
Top with 0.3 cups parmesan, finely grated while beans are still hot so it softens slightly. Serve immediately.