Preheat your oven to 200°c/180°c fan / gas mark 6
Thaw your frozen prawns by placing them in a colander and rinsing with cold running water (don't use hot!) until they are soft. Season them with a little salt & pepper and leave in the fridge while you prepare the rice.
Warm an oven safe casserole dish over a medium-high heat and melt half the butter
Add the onions and fry until they are soft but not browned - about 5 minutes
Add the garlic and cardamom pods, stir and cook for another minute
Stir in the rice and the turmeric then pour in the stock and bring it to the boil
Pop on the lid, wait for about a minute to allow some steam to build up, then put the pot in the pre-heated oven for 15-20 minutes
Once your rice has cooked - it should have soaked up the liquid without being dry and be al-dente to bite but not chewy- you can cook the prawns and peas
Melt the remaining butter in a frying pan and add the frozen peas (no need to thaw). Stir them over a medium to high heat for about 3-5 minutes until they are cooked. Then add your thawed prawns, cooking for about a minute until they are warmed through.
Pick out the cardamom pods from the rice - they should have risen to the top to make it nice and easy for you.
Stir the prawns, peas and crème fraîche into the rice. Add salt & pepper to taste.
Serve, garnished with the chopped herbs, and enjoy!