PreparationFor mayonnaise: Mix all ingredients in small bowl. Cover and chill. Do ahead: Can be made 2 days ahead. Keep chilled. For burgers: 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. Do ahead: Can be made 1 day ahead. Cover and chill. For onions: 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. *Shopping tip: Look for pickled okra where the pickles and relishes are displayed.
PreparationFor mayonnaise: Mix all ingredients in small bowl. Cover and chill. Do ahead: Can be made 2 days ahead. Keep chilled. For burgers: 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. Do ahead: Can be made 1 day ahead. Cover and chill. For onions: 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. *Shopping tip: Look for pickled okra where the pickles and relishes are displayed.