Turn the oven on to 400 degrees, and line a large baking pan with parchment paper. You might need two pans depending on how many green beans you have and the size of your oven.
Trim the ends off the green beans and place them in a large bowl. We'll usually snap the ends off with our hands, but using a knife to cut them works too.
Add your green beans, oil, garlic, parmesan, salt, and pepper to a bowl and toss until fully coated.
Then pour your green beans onto your prepared baking pan, and spread them out in a single layer. (It's ok if a few beans are overlapping.) Place them in the oven and roast for 15-20 minutes.
Squeeze a little lemon over the top (if desired) and serve immediately!