We upgraded this classic holiday side dish with a tangy balsamic glaze and sweet caramelized onions.
Cook beans in boiling water to cover in a large saucepan until tender-crisp, 3 to 4 minutes. Plunge beans into ice water to stop the cooking process; drain well.
Heat 1 tablespoon of the butter in a large nonstick skillet over medium. Add pearl onions; cook, stirring often, until lightly caramelized and tender, about 10 minutes. Add garlic, and cook, stirring constantly, until fragrant, about 1 minute. Add balsamic vinegar and sugar, and cook until reduced to a light syrup consistency, 3 to 4 minutes. Add beans, salt, pepper, and remaining 1/2 tablespoon butter, and toss to coat. Serve immediately.