Gently fry the onion and garlic in oil until soft and golden. Add rice and cook, stirring, for a minute before adding the tomatoes, water and lemon juice. Season with salt and pepper, bring to the boil, reduce heat to a low simmer and cook uncovered for 10 minutes.
Add the prawns, tucking them in the rice, and continue to cook for another 5 minutes or until rice is just soft.
Remove from heat and allow to cool slightly, then crumble over the feta cheese, mint and parsley and serve with lemon wedges.