Preheat oven to 425°F. Line baking sheet with parchment paper, or coat with cooking spray.

Bring large pot of salted water to a boil. Add green beans and red onion, and blanch for 3 minutes. Drain, then pat vegetables dry with paper towels.

Whisk together oil, lemon juice, mustard, salt, and pepper in a large bowl. Add beans and onion slices; toss to coat.

Group beans and onion slices into 8 bundles (about 10 to 15 beans each), and tie each bundle with an 8-inch piece of kitchen twine. Place bundles tie side down on prepared baking sheet.

Bake for 7 to 10 minutes, or until bean ends begin to brown. Flip bundles with spatula so tie side is up, and bake for 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.