This honey-mustard dipping sauce spiked with plenty of onion is delicious with oven-fried chicken or slathered on a steak sandwich. The secret to a creamy, emulsified dressing is mustard. Mustard helps thicken liquid sauces by absorbing some of the liquid and allows the suspension of one liquid in another. If Vidalia onions are unavailable, use another sweet onion, such as Walla Walla or Texas Sweet. Half of one gives this sauce big onion flavor; if you like something more subtle, try a quarter of an onion instead.