Cook vinegar in a small saucepan over medium heat 9 minutes or until syrupy and reduced to 3 tablespoons; set aside.
Toss beans with oil in a large bowl. Spread beans on a large rimmed baking sheet; sprinkle with salt and pepper.
Bake at 475° for 15 to 18 minutes or until charred in appearance (do not stir). Toss hot beans with balsamic syrup. Serve immediately.
Fix it Faster: Purchase prewashed, trimmed green beans available in the produce department.
Make Ahead: The balsamic syrup can be made a day ahead and stored at room temperature. Make a few extra batches of the syrup and consider it for gift giving.