Peel and core the apples and cut into small cubes. Thaw blueberries if necessary if frozen ones are used. Heat the butter in a saucepan. Add the apples and blueberries with the sugar, cinnamon and lemon juice and simmer over a medium heat with the lid on for 15 minutes.
During the cooking time, fill two small glasses with boiling water and dry them with a kitchen towel.
Puree the sauce and pour it into the glasses while hot. Close the jars and turn them upside down for 2 minutes. Store in the refrigerator until use.
Goes great with pancakes or pancakes, but goes well with any kind of dessert.