Place raspberries, sugar, water, salt and lemon juice in a small saucepan and simmer over low heat until sugar has dissolved and raspberries are breaking down.  Do not boil.

Transfer mixture to a blender and puree until smooth.

Using a mesh strainer, strain mixture into a bowl, removing all seeds.  

Place bowl of puree to fridge and leave for at least 1 hour to cool before using.