Spiced lamb burgers with herbed yoghurt

Name: Spiced lamb burgers with herbed yoghurt
Ingredients: 1 small onion, quartered; 4cm/1½ in piece fresh ginger; 4 large garlic cloves; 20g/¾oz fresh coriander, stalks and leaves; 2-3 green chillies, chopped or ½-1 tsp red chilli powder; 450g/1lb lamb mince; ¾ tsp ground cumin; 1½ tsp garam masala; 1 tsp salt, or to taste; 1 large free-range egg; 2 slices thick-cut white bread, processed to crumbs in a food processor; 2 tbsp oil, for greasing; 200g/7oz greek-style yoghurt; 30g/1oz fresh coriander leaves; 15g/½oz fresh mint leaves, or 1 tbsp good quality dried mint; 1-2 green chillies, finely chopped (optional); Salt, to taste; ½ tsp freshly ground black pepper; 6 hamburger buns; 1 onion, sliced crosswise into large rings; 2 ripe plum tomatoes, sliced crosswise; Crunchy lettuce leaves