Bring a large saucepan of lightly salted water to a boil. Drop in the green beans and cook, uncovered, 5 to 7 minutes or until tender but still crisp. Drain. (Can be made ahead to this point. Rinse with cold water to stop the cooking. Place in a resealable plastic bag and refrigerate.) Dry the saucepan and return it to the stove. Toast the mustard seeds over low heat just until they pop, about 1 minute, stirring to prevent scorching. Add the honey and vinegar and cook until the sauce boils and becomes syrupy. Add the green beans and toss.