Season the prawns with turmeric and chilli powder then set aside. Toast the cashews, cardamom, cinnamon and peppercorns in a frying pan over medium heat until fragrant, about 7 minutes. Remove from the frying pan and set aside.
Heat the sunflower oil in a large frying pan over medium-high heat. Add the onion, garlic and ginger; cook and stir until the onion has softened and begun to lose its red colour, about 5 minutes. Stir in the prawns and toasted spice mixture along with the salt, garam masala, bay leaf, tomatoes and capsicum. Cook and stir until half of the prawns have started to turn pink then pour in the coconut milk. Cover and bring to a simmer.
Reduce heat to medium-low and gently simmer until the prawns are opaque and the vegetables are tender, about 5 minutes.