Best burger ever

Name: Best burger ever
Prep time: 45 min
Cook time: 15 min
Total time: 1 h
Cuisine: American
Ingredients: 1 small onion, finely chopped; 1 garlic clove, finely chopped; 6 x thyme sprigs, leaves picked and chopped; 600g beef mince (minimum 15% fat); 1 egg yolk; 1½ tsp English mustard, plus extra for spreading; 50g Parmesan, finely grated; 4 rashers smoked streaky bacon; 4 thin slices mature Cheddar; 4 brioche burger buns, cut in half; 6 iceberg lettuce leaves, chopped; 1 beef tomato, sliced into 8; 4 tsp tomato ketchup; ½ cucumber, thinly sliced; 50ml white wine vinegar; 2 tsp caster sugar; ½ tsp mustard seeds; ¼ white cabbage, sliced; ½ carrot, grated; ¼ onion, finely chopped; ½ tbsp mayonnaise; 1 tbsp crème fraîche; few chives, chopped