Place a couple of saucers into the freezer and sterilise your jars; I do this by washing them in hot, soapy water, rinsing well then placing the jars, and their lids, in the oven on a low heat until needed.
Place the pears, apples, lemon zest and juice, ginger and water into a deep, heavy bottomed saucepan. Bring up to a simmer and simmer gently for about 15 minutes, until the fruit is tender.
Stir in the sugar and stem ginger and heat gently until the sugar has completely dissolved.
Turn up the heat and cook at a rapid boil for about 10 minutes, or until it reaches 105C on a thermometer; stir regularly to prevent it from catching on the bottom.
Remove from the heat and place a teaspoonful of the jam on one of the cold saucers; pop it in the fridge for a minute then push at it with a finger, if the surface of the jam wrinkles then it is ready; if not then return the pan to a rapid boil for another five minutes then remove from the heat and check again.
Allow the jam to cool for 5-10 minutes, then ladle it into hot sterilised jars, seal and allow to cool. Once opened keep refrigerated.