Prolific cookbook author James McNair has been chief judge of Sutter Home Winery's burger cook-off since it began in Napa in 1990.
Mix all ingredients in small bowl. Cover and chill.
Toss first 4 ingredients in large bowl. Add beef; blend gently. Shape mixture into six 1/2-inch-thick patties. Transfer patties to small baking sheet.
Prepare barbecue (medium-high heat). Toss onions and next 3 ingredients in large skillet. Place skillet on grill; cook until onions are golden, stirring often, about 25 minutes. Remove from grill; season with salt and pepper.
Brush grill rack with olive oil. Grill buns, cut side down, until golden, about 2 minutes. Transfer buns to work surface. Grill burgers until brown on bottom, about 3 minutes. Turn over; sprinkle with cheese. Grill until burgers are cooked to desired doneness, about 3 minutes for medium. Place some onions, then burger, on each bun bottom. Top each with okra and watercress. Spread mayonnaise on cut side of bun tops; place on burgers. Serve with remaining mayonnaise.
Serving Size 1 Burger
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.