Measure balsamic vinegar into a small saucepan and set over medium heat. Cook for 5 to 8 minutes or until vinegar has reduced and turned syrupy enough to coat a spoon. (If vinegar starts boiling during the cooking time, reduce heat enough to bring it down to a simmer.) Set aside.
In the meantime, preheat oven to 475°F. Place green beans in a large bowl. Drizzle with olive oil and toss until evenly coated. Spread out on a rimmed baking sheet and sprinkle with garlic salt and pepper.
Roast for 15 to 18 minutes, stirring once halfway through, or until the edges of the beans are starting to caramelize and char.
Drizzle green beans with balsamic reduction (to taste) and toss to coat. Transfer to a bowl or serving platter and serve warm.
Balsamic syrup may be made a day ahead of time and stored at room temperature.
Alternatively, you may purchase a bottle of pre-made balsamic syrup.