Preheat the oven to 400 degrees F (204 degrees C).
In a large bowl, toss together the green beans, 2 tablespoons olive oil, garlic powder, sea salt, and black pepper.
Arrange the green beans in a single layer on a baking sheet. Roast for 10 minutes.
Meanwhile, thinly slice the garlic, then toss it with the remaining ½ tablespoon olive oil.
Take the pan of green beans out of the oven, sprinkle on the sliced garlic and grated parmesan, toss and arrange in a thin layer again. Continue roasting for 5-10 more minutes (mine took 8 minutes), until the garlic starts to caramelize and the green beans are crisp-tender with some browned spots.