1 Little Gem, leaves separated
150g/5陆oz raw beetroot, peeled and grated
1 large carrot, peeled and coarsely grated
2 sticks celery, very finely chopped
6 large gherkins, thinly sliced
陆 lemon, juice only
1 tbsp chopped fresh marjoram
1 small onion, finely chopped
3 tbsp chopped flatleaf parsley
2 spring onions, chopped
1 tbsp sun-dried tomato paste
2 large beefsteak tomatoes, thickly sliced
2 tsp Dijon mustard
1 tbsp hot horseradish sauce
4 tbsp full-fat mayonnaise
salt and freshly ground black pepper
50g/1戮oz fresh white breadcrumbs
2 tbsp oil
dash sugar
1 free-range egg yolk
500g/1lb 2oz lean beef mince
6 burger or brioche buns, cut in half