Drain pineapple, reserving juice. Cut slices in half; set aside. In a jar with a tight-fitting lid, combine the oil, honey, vinegar, soy sauce and reserved pineapple juice; shake well. , Refrigerate for at least 30 minutes. Combine the romaine, onion slices and pineapple slices in a serving dish. Drizzle with dressing; toss to coat.