Set the oven to 425°F. Line a baking sheet with parchment paper or coat with cooking spray.
Bring a large pot of salted water to a boil. Add green beans and cook for 3 minutes, until bright green and crisp. Drain and rinse with cold water. Pat dry with paper towels and transfer to a large bowl.
In a small bowl, whisk together the olive oil, lemon juice, Dijon mustard, salt, garlic powder, and black pepper. Add the green beans and stir gently to coat.
Group the beans into 8 bundles (about 8–10 beans each), and tie each bundle with a 12-inch piece of kitchen twine. Place the bundles, tie side down, on the prepared baking sheet.
Roast for 6 minutes, flip each bundle, and roast another 6 minutes, until the tips are lightly browned.
Transfer to a serving plate, sprinkle lightly with lemon zest, and serve hot.