Preheat oven to 350 degrees. Put raspberries and sugar into 2 pie plates (disposable kind works best) so the fruit will spread out rather than piled up. Put the pans in oven for 20 to 25 minutes until they are really hot.
Take them out of oven carefully and add the sugar to the raspberries. As you do so, you'll find the fruit turns into a molten ruby red river. Pour this into the cleaned and waiting jar. Fasten with the lid and leave it to cool before storing in fridge.