While the pastry is in the fridge, place the pancetta on a low heat in a pan. Dice the onion into small pieces. Add the onion to the pancetta and saute for 5-10 mins until cooked. There is no need to add oil into the pan as the pancetta releases fat as it cooks. If you find there is too much fat in the pan from the pancetta, use a piece of kitchen roll to dab on the fat to soak some up. Once cooked, place a couple pieces of kitchen roll in a bowl and scoop in the pancetta and onion. The kitchen roll will soak up some of the oils/fat. Leave to cool.