Heat the oil in a wide frying pan over a medium heat. Fry the onion with a pinch of salt for 3-4 mins until slightly softened. Stir in the garlic, chilli (if using), oregano and purée and cook for a further 2 mins. Turn the heat up to high and pour in the wine. Leave to reduce by half, about 3-5 mins, then stir in the cherry tomatoes, orzo and stock. Bring to a simmer and cook, stirring occasionally for 5-7 mins, until the orzo is almost tender. Stir in the prawns and cook for 2-3 mins until opaque and cooked through. Season and scatter over the parsley and feta. Serve the lemon wedges on the side for squeezing over.