1. Make the pickled red onions by bringing the 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. Repeat the process twice until all the onions have been cooked. Pour the pickling juice over the cooked onions, cover and refrigerate until serving.
2. Make the barbecue dressing by combining the crème fraîche and barbecue sauce in a bowl and refrigerate until serving.
3. Make the cabbage salad by stirring together the cabbage, lemon juice, salt and pepper.
4. Cut the cucumber into 12 wedges. Heat a grill pan over medium-high heat. Sear the cucumber pieces for about 2 minutes on each side. Sprinkle with 1/2 tsp salt and remove from the heat.
5. Form the meat into 4 patties and season with 1/2 tsp salt and pepper. Grill the patties for approx. 3 min. on each side, until cooked as desired.
6. Toast the buns and spread them with the barbecue dressing, then layer with a burger patty, oxheart cabbage salad, seared cucumber and pickled onions. Serve the burgers with grilled corn on the cob.