Heat 1 tablespoon of oil in a large stock pot over medium-high heat. Crumble turkey into the pot, stirring with a wooden spoon to break apart as much as possible. Season with taco spice mix, coriander, oregano, chilli flakes and tomato puree, and mix until meat is evenly coated with seasonings. Continue cooking, reducing heat if necessary, until turkey is well browned.
Pour in beef stock, and simmer to reduce liquid slightly, about 5 minutes. Add passata and green chillies, and continue cooking at a moderate simmer for ten minutes. Adjust the thickness at any time you feel necessary by adding water.
While chilli is still cooking, heat one tablespoon of oil in a large frying pan over medium-high heat. Cook onion and green pepper, stirring occasionally for 5 minutes, or until onion is translucent and pepper is lightly browned. Add onion and pepper to the chilli, and continue cooking at a very low simmer.
In the same frying pan, heat the remaining tablespoon of oil over medium-high heat. Add the courgette, and cook stirring occasionally, for 5 minutes, or until lightly browned. Add the courgette to the chilli, reduce heat, and continue cooking 15 minutes more. Again, adjust the consistency with water as needed.
Ladle chilli into serving bowls. Top with soured cream, spring onion and Cheddar cheese, and serve.