Put the pancetta in a large, deep frying pan over a medium-high heat and fry until crisp and golden. Remove from the pan with a slotted spoon, transfer to a plate and set aside. Reduce the heat, add the onion along with a pinch of salt and fry in any leftover fat for 5 mins. Add the garlic and continue to fry for a further 3 mins.
Increase the heat and stir in the orzo. Fry for 2 mins, stirring constantly, then pour in 500ml stock and bring to a simmer. Add more stock by the ladleful, stirring constantly, as the orzo absorbs the liquid (similar to cooking a risotto).
After 5 mins, stir in the petit pois and bring the mixture to a simmer. Continue to add the remaining stock, stirring, as the mixture cooks. Simmer for 10 minutes, or until the orzo is cooked through. Add the lemon zest and juice, basil, parmesan and fried pancetta. Season to taste, then cover and leave to rest for 2 mins. Serve with extra parmesan sprinkled over the top.