Preheat oven to 450 degrees. Line a rimmed baking sheet with aluminum foil. Place the green beans and onion wedges in the prepared baking sheet and toss with the olive oil, salt and pepper. Spread into an even layer. Roast for 10 minutes.
Combine the balsamic vinegar with the honey in a small bowl. When the beans and onions have roasted for 10 minutes, remove from the oven, pour the vinegar/honey mixture over the beans and onions and toss with tongs to coat. Return to oven and continue to roast for 12-15 minutes or until the onions and beans are dark golden brown in spots and the beans are starting to shrivel. Transfer to a serving dish and serve.