Toast the cumin seeds with the peppercorns till fragrant. Remove and crush in a mortar and pestle. Combine with the cayenne, mixed herbs and salt and rub all over the prawns. Set aside for 5 minutes.
Heat up 1 teaspoon of oil in a pan till hot. Place the prawns in the pan in a single layer (you may need to do this in batches). As soon as the prawns turn pink on one side – a minute/two, turn over and allow to cook on the other side. Once that side turns pink, remove and set aside in a bowl. Repeat greasing of pan and frying of prawns till all are done.
Let the prawns cool down then remove the shells – do not discard the shells but set aside,separating flesh from shells.
In a large pan, fry the chopped onions on medium to low heat till just caramelized. Add the shrimp shells to the onions and cook for 4-5 minutes, stirring continuously. Then add 3 cups of water, the chopped tomatoes and sliced garlic. Allow to cook for 10 minutes, stirring now and again.
Put the prawn shells – onion mixture into a food processor and blitz till the shells are chopped up. Then add the yellow bonnet – adding small pieces and tasting, till the right heat level is achieved. Pass the pureed mixture through a sieve twice – discard the gritty shell bits and reserving the smooth puree
Make a roux on medium – low heat in the pan by combining the butter and flour. Allow to cook, stirring for 2-3 minutes. The remove the pan off the heat for a minute while you add the fine shrimp puree/liquid back to the pan, slowly stirring till thickened - if too thick, thin with some water. Allow to heat up gently. Once gently bubbling, add the cooked, shelled prawns – stir well to combine and let just heat up for a minute or two till warm all over
Add the cream (at the last minute, and if using) and serve with freshly boiled white rice and some chopped chives/coriander to top.