Place the Jazz Apples and quinces in a large pan with 150g of the sugar and enough water to just cover them. Bring to the boil stirring occasionally and then reduce the heat and simmer with the lid on until the Jazz Apples and quinces are soft and mushy.
In a smaller saucepan place the remaining sugar(100g), about 100ml of water and the thyme. Dissolve the sugar in the water over a low heat and them bring to the boil. Allow to bubble away for a few minutes to create a sugar syrup. Remove from the heat and allow to cool. Remove the thyme sprigs.
While this is cooling take a large bowl and over it place a wire sieve. Pour the slightly cooled fruity contents into the sieve and with the back of a spoon work through the sieve until all you have left are the skins and seeds, which can them be discarded. Squeeze the lemon juice into the bowl of fruity gloop. Once the sugar syrup is completely cold, mix this into the Jazz Apple and quince mixture.
Put this mixture into a Tupperware container and put into the freezer. Every ½ hour or so remove from freezer and whisk up to prevent ice crystals forming. Reseal and return to the freezer. On the second beating add the egg white and mix thoroughly. Repeat this whisking process until ice crystals are no longer forming (be sure to get into the corners of the container each time). Return to the freezer and allow to fully freeze. If you have an ice cream maker of course you can pour the mixture straight in there and let it do the work!
Remove from the freezer for about 15 minutes before serving. I serve this dressed with assorted edible flowers and leaves – thyme, lavender, marjoram and mint.