Drizzle a little olive oil in a frying pan and bring to medium heat. Add the defrosted shell-on king prawns or jumbo prawns and gently cook for 3-4 minutes or until the shells have turned pink and opaque. Remove the prawns and set aside to cool.
Reserve 2 whole shell-on prawns and remove the heads and shells from the rest of the prawns keeping the prawns and shells separate. Run a sharp knife down the back of each prawn (including the shell-on prawns) and remove and discard the digestive tract.
Place the pan back on the heat and add the chopped leek, carrot and fennel. Fry for 7-8 minutes until softened. Add the garlic and fry for further 2 minutes.
Stir in a tablespoon of Harissa paste and add the orange juice and pinch of saffron. Reduce the orange juice until almost dry. Add the passata, red pepper, fish stock and the reserved shells and simmer for 10-12 minutes. Set aside to cool slightly.
Transfer the contents of the pan, including the shells, to a high-speed blender and process until completely smooth. Pour the contents back into the pan passing it through a sieve using the back of a ladle to help it through.
Bring a large pan of salted water to a boil and cook the pasta for 9 minutes. Drain and set aside reserving a little water.
Bring the sauce pan back up to a simmer and add the pasta and herbs and stir. Add the de-shelled prawns and cook for up to another minute until the prawns are re-heated.
Divide between 2 pasta bowls and present with a whole shell-on king prawn on top with a garnishing of fresh oregano and grated parmesan.