Cook green beans in boiling salted water 5 to 7 minutes or until crisp-tender. Plunge beans in ice water to stop the cooking process; drain and set aside.
Heat oil in a large skillet or sauté pan over medium heat; add onion and garlic, and sauté 15 to 20 minutes or until tender and onion starts to caramelize. Stir in green beans, salt, and pepper; sauté 2 to 3 minutes or until beans are heated. Serve hot.
Make-Ahead Note: Place cooked green beans in a large zip-top plastic bag, and store in refrigerator up to 1 day. When ready to use, simply toss in with caramelized onions, and sauté until warm.