I usually make these turnovers with homemade rough-puff pastry (also called blitz pastry) because it tastes better, but I won't blame you if you use commercial puff pastry. It's worth seeking out all-butter puff, and it's even better if you buy it in a block, rather than rolled out sheets, which are too thin. My favourite jam to use as a filling is homemade mango jam - it has a bright, summery taste and the mango flavour is very pure. You can make it with other types of jam, but try the mango...