Creamy Chicken, Mushroom and Green Chilli soup

Name: Creamy Chicken, Mushroom and Green Chilli soup
Prep time: 25 min
Cook time: 3 h 45 min
Total time: 4 h 10 min
Cuisine: British
Ingredients: 1 large roasted chicken carcass with some of the meat still on*; 1 brown onion (chopped in half (no need to peel)); 2 medium carrots (chopped in half (no need to peel)); 1/2 tsp salt; 1/2 tsp black pepper; Water to cover; 2 tbsp. butter; 1 brown onion (peeled and chopped); 6 chestnut mushrooms (sliced (Crimini or Portabello work just as well too)); 1 green chilli (sliced (or two if you like it hotter)); 2 cloves garlic (peeled and minced); 2 tbsp plain (all-purpose) flour (**see notes below for gluten free); 800 ml (3 1/2 cups) of your prepared chicken stock; 3 tbsp double/heavy cream; 1 tsp lemon juice; Chopped spring onions (scallions and chopped green chilli)