Preheat your oven to 200°c/180°c fan / gas mark 6/400°f
Season the prawns with a little salt & pepper and leave in the fridge while you prepare the rice.
Warm an oven safe casserole dish on the hob over a medium-high heat and melt the butter.
Add the onions and cook, stirring occasionally, until they are soft but not browned - about 5 minutes.
Use the handle of a knife to gently crack the cardamom pods and add them with the garlic to the pan, stir and cook for about 2 minutes until the garlic is fragrant.
Add the rice and turmeric and stir well for 1-2 minutes, allowing the rice to toast lightly and get coated in the butter and spices. Pour over the stock and bring it to the boil without stirring.
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 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) 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 add the pot back into the oven for 5 minutes to allow the peas and prawns to warm through. (If using raw prawns make sure they are fully cooked and pink - you may need a few extra minutes.)
To serve, stir the crème fraîche into the rice. Add salt & pepper to taste and garnish with the chopped herbs.
Notes
To defrost frozen prawns/shrimp - Thaw frozen prawns by placing them in a colander and rinsing with cold running water (don't use hot!) until they are soft. Alternatively defrost overnight in the fridge.