Marinate the chicken: in a large bowl, mix 1000 g chicken thighs with 250 g plain yogurt, 1 finely chopped onion, 3 tbsp ginger-garlic paste, 2 tbsp lemon juice, 2 tbsp biryani masala, 1 tsp red chili powder, 0.5 tsp ground turmeric, 1 tsp ground cumin, 1 tsp ground coriander, 2 slit green chilies, half the chopped mint and coriander, and salt to taste. Cover and refrigerate for at least 4 hours or overnight.