Place chicken, stock, water, 2 cloves garlic, pepper and oregano in a medium saucepan. Simmer, uncovered, 25 minutes.
Remove chicken from pan. Discard bones. Cut chicken into large chunks. Strain and reserve stock.
Heat oil in a large saucepan over medium heat. Crush remaining garlic and add to pan with the spring onions. Sauté until softened, 5 minutes. Add chillies, tomato and strained stock. Simmer, partially covered, 15 minutes. (Recipe can be made ahead up to this point.)
Add chicken, lime juice and toasted tortilla strips. Simmer 5 minutes. Garnish with coriander.