Trim the courgettes and peel thinly in alternate strips. Cut into thick matchsticks.
Put the meat in a heavy pan and cover. Let it absorb its own juices on low heat, then add the onions and fat. Fry, uncovered, stirring occasionally for 4-5 minutes until the onions turn golden, add the tomato purée and stir for a further 1 minute.
Put the courgettes on top of the meat, then the