In a large high-sided pan toast the spice blend on medium heat for 3-4 minutes, stirring constantly to prevent it from burning. Set aside.
In the same pan heat the oil on high and fry the onion until golden brown. Lower the heat to medium and add the ginger, garlic and toasted spice. Cook for 3-4 minutes, stirring continuously until it forms a paste-like consistency.
Slowly add the passata, followed by the chicken stock, then stir through and bring to the boil.
Lower the heat and add the chicken, carrots and cauliflower. Stir so the pieces are evenly coated, then cover with the lid and cook on low heat for 45 minutes. Stirring every 10 minutes to ensure even cooking.
Once cooked, stir through lemon juice and season with salt to taste.
Combine the Nigella seeds with the chutney, lemon zest and enough juice to loosen the mixture slightly. Swirl through the yoghurt to accompany the curry, then top the dish with fried curry leaves.