1. Pickled red onions: Bring the vinegar and sugar to a boil in a heavy-bottomed saucepan. Add ⅓ of the onions and boil for about 2 min. Remove the onions from the saucepan with a slotted spoon and place them in a bowl. Continue until all the onions are cooked. Pour the pickling juice over the onions, cover and refrigerate until serving.
2. Barbecue dressing: Combine the creme fraiche and barbecue sauce in a bowl and refrigerate until serving.
3. Oxheart cabbage salad: Toss together the oxheart cabbage, lemon juice, salt and pepper. Season to taste.
4. Split the cucumber lengthwise and scrape out the seeds. Cut the cucumber into 12 pieces in all.
5. 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 high heat.
6. 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.
7. Warm the buns and spread them with the barbecue dressing, then layer with a burger patty, oxheart cabbage salad, cucumber and pickled onions. Serve the burgers with grilled corn on the cob.