Bring a large pot of lightly salted water to boil. Place green beans in the water, and cook about 3 minutes. Remove from heat, and drain in a colander. Place under cold water until no longer hot. Drain, and pat dry.
Melt 1 tablespoon butter in a medium skillet over medium heat. Stir in the onion and garlic, and saute until onion is tender. Melt remaining butter in the skillet, and mix in the green beans. Stir in the vinegar and broth. Season with salt and pepper. Mix in cilantro. Cover, reduce heat, and simmer 15 minutes, or until green beans are tender.