Bring vinegar and sugar to a boil in a heavy-bottomed saucepan. Add ⅓ of the onions and boil them for about 2 min.Remove the onions from the saucepan with a perforated ladle and place them in a bowl. Continue until all the onions have been cooked. Pour the pickling juice over the onions, cover and refrigerate until serving.
Combine the crème fraîche and barbecue sauce in a bowl and refrigerate until serving.
Stir together the oxheart cabbage, lemon juice, salt and pepper. Season to taste.
Split the cucumber lengthwise and scrape out the seeds. Cut the cucumber into 12 pieces in all. Form the meat into 4 patties and season with half the salt and pepper. Heat a grill pan until nice and hot – about 4 min. over a high heat. Sear the cucumber pieces for about 2 min. on each side. Sprinkle with the rest of the salt. Then grill the patties for approx. 3 min. on each side.
Warm the buns and spread them with the barbecue dressing, then layer with a burger patty, a Castello Blue Cheese Slice, oxheart cabbage salad, cucumber and pickled onions. Serve the burgers with grilled corn on the cob.