.Turn the slow cooker on high to preheat for five minutes, with the lid on.
Heat the butter and sugar in a saucepan on low heat, until the butter has mostly melted. Remove from heat.
Stir in the lemon and orange zest and juice, then the eggs and vanilla extract.
Put the flour, baking powder and ground almonds in a bowl.
Pour in the mixture from the saucepan. Beat until smooth until you have a runny batter.
Pour one third of the mixture into a round Pyrex dish that fits in your slow cooker. Drop one third of the berries evenly onto the batter without pushing them in. Repeat this twice.
Place the Pyrex dish in your slow cooker, pour one to two inches of boiling water around the Pyrex dish, put the lid on the slow cooker, and cook on high for 2-3 hours until a toothpick comes out clean.