In a pan add the mixed berries, together with the lemon juice, lemon zest, vanilla extract and the sweetener of choice.
Put the pan over a low heat and cook by stirring frequently.
Prepare the cornstarch by dissolving it in 3 Tablespoon of water.
Once the sauce starts to bubble add the mixture of water and cornstarch in the pan and stir.
Cook for another 2 to 3min on low heat.
The sauce should start to thicken.
Once ready remove the pan from the heat and let it cool for 5min before serving.
I like serving my rice pudding in jars or glasses with layers of the mixed berries sauce in between.
For decoration, top it with berries or any other fruit of choice. (optional)