1. Preheat oven to 425°F. Line baking sheet with parchment paper, or coat with cooking spray.
2. Bring large pot of salted water to a boil. Add green beans and red onion, and blanch 3 minutes. Drain, then pat vegetables dry with paper towels.
3. Whisk together oil, lemon juice, mustard, salt, and pepper in large bowl. Add beans and onion slices; toss to coat. 4. Group beans and onion slices into 8 bundles (about 10 to 15 beans each), and tie each bundle with 8-inch piece of kitchen twine. Place bundles tie side down on prepared baking sheet. Bake 7 to 10 minutes, or until bean ends begin to brown. Flip bundles with spatula so tie side is up, and bake 7 to 10 minutes more, or until all bean tips are brown. Transfer bundles to serving plate, and garnish with lemon slices and Hawaiian salt, if using.