These look like little flying saucers! Pick when small - around 3". Scoop out the seeds in the centre and stuff or chop and use like courgette or in risotto etc. They take a little longer to cook than courgettes.
Grow like courgettes. Can be sown up to early June. Early sowings (March-April) should be made under glass and seedlings potted on into good sized pots (1 to 2 litre depending on how long they need to be kept potted) as they should not be planted outside until all danger of frost is over.