Bulgogi burgers with quick-pickled cucumbers

Name: Bulgogi burgers with quick-pickled cucumbers
Cook time: 30 min
Total time: 30 min
Cuisine: Asian, Korean
Ingredients: 200ml rice vinegar; 75g caster sugar; 1 tsp ground turmeric; 1 or 1 medium cucumber Lebanese cucumber thinly sliced; 4 tsp sunflower or vegetable oil; 4 brioche buns or soft white burger buns; 5 tbsp mayonnaise; 1 tsp toasted sesame oil; ½ small white cabbage cored and finely shredded; 2 tsp soft light brown sugar; 4 tsp light soy sauce; 500g beef mince; 50g soft breadcrumbs; 2 cloves garlic crushed; grated to make 2 tsp ginger; 2 tbsp gochujang plus extra for serving (optional); 2 tbsp toasted sesame seeds; a generous grinding black peppercorns