I used veal for this recipe. To cook the veal, after the first whistle on full flame, I reduced the flame to lowest and cooked for 8-10 mins. I guess, you can use Mutton instead of beef and follow the same recipe. You can add more coriander and mint leaves, if you like. Also, you can add a tbsp of lemon juice along with rice, if you like.
Biriyani keeps well in the fridge for 3 days.