For the curry, halve and scoop flesh of tamarillos into a food processor and blend to a pulp.
Heat oil in a large, deep-sided frying pan and cook the onions for 5 minutes. Add the ginger, garlic and chilli flakes and cook for 1-2 minutes. Stir in the garam marsala, brown sugar and tomato paste and cook another 30 seconds or so.
Add tamarillos and tinned tomatoes and bring to a boil then reduce heat to medium and cook, stirring often until sauce thickens, 6-8 minutes. Add coconut milk and cook, stirring occasionally, until sauce thickens, approx 20 minutes; taste and season with sea salt. Add chicken and reduce heat to low. Cook, partially covered, until chicken is cooked through, 10-12 minutes. Check seasoning and add more salt if needed.
Stir yoghurt, a big pinch of salt, lemon juice and coriander in a small bowl.
Drizzle yogurt sauce over curry and top with extra coriander. Serve with rice.