By Karla Walsh, CNN (CNN) — On the eve of the nation’s 250th anniversary, families and friends across the United States are preparing to mark the holiday with a summer cookout. A potluck salad that’s been a showstopper at Meggan Hill’s family gatherings for more than 50 years should be on your picnic table. The creamy side dish is sure to get your guests’ taste buds fired up. “Midwestern food is often judged or almost dismissed because some of t…