Position racks in the top and bottom thirds of the oven and preheat to 375°F In a bowl, combine the dates and breacrumbs, working the mixture through your fingers to separate. Mix in the beef. Mix in the chopped onion, bacon, egg, worcestershire and vinegar; season with salt and pepper. Divide into 4 mounds and, on a baking sheet, shape into loaves. Place on the upper rack and bake for 45 minutes.
Meanwhile, on another baking sheet, toss the slices onions with the butter and season with salt; spread out evenly. Bake ont he lower rack, stirring occasionnally, until golden, about 30 minutes. Serve the meatloaves with the onions.