Heat the olive oil in a large pot over medium heat. Sauté the onion, carrots, and celery for 6-8 minutes until soft.
Stir in the garlic and dried herbs, cooking for 1 further minute.
Sprinkle the plain flour over the vegetables and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste.
Slowly pour in the chicken stock while whisking constantly to prevent lumps.
Add the cubed potatoes.
Bring to a gentle boil, then reduce heat to a simmer. Cover and cook for 10-12 minutes until the potatoes are tender.
Turn the heat down to low. Stir in your pre-cooked shredded chicken, the milk (or cream), and the grated mature Cheddar.
Stir gently for 2-3 minutes until the soup is hot and the cheese is completely melted.
Season with sea salt and black pepper to taste. Ladle into bowls and garnish with sliced spring onions.