1 Little Gem lettuce, leaves separated
1 ripe avocado, peeled, stone removed, chopped
1陆 tbsp chopped fresh coriander leaves
1 garlic clove, finely chopped
陆 lime, juice only, plus extra to serve
salt and freshly ground black pepper
1 tbsp olive oil
3 tbsp plain yoghurt
150g/5陆oz lean beef mince
2 flour tortillas