Prep the green beans: Preheat the oven to 400°F. Wash the green beans, dry them thoroughly, and trim the stem ends.
Season: Place the green beans on a large sheet pan. Drizzle with olive oil and add the garlic, Parmesan, kosher salt, and black pepper. Toss until evenly coated, then spread into a single layer.
Roast: Roast for 20–25 minutes, tossing halfway through, until the green beans are tender with lightly browned edges.
Finish: Remove from the oven and squeeze fresh lemon over the top. Add extra Parmesan and black pepper, if desired, and serve immediately.