purple cauliflower spring soup with crème fraiche

Name: purple cauliflower spring soup with crème fraiche
Ingredients: 3 tablespoons olive oil; 1 red onion, peeled and finely chopped; 1 carrot, peeled and chopped; 2 celery stalks, chopped; 2 garlic cloves, peeled and finely chopped; 1 russet potato, peeled and finely chopped; 1 large purple cauliflower, cut into florets; 5 cups chicken stock; 1 cup Greek plain yoghurt; 3 tablespoons apple cider vinegar; Sea salt and freshly ground black pepper; Crème Fraiche, to garnish; 1 tablespoon of chopped flat Italian parsley