Add the frozen berries sugar, and lemon juice to a medium sized pot or saucepan. Cook the mixture over medium heat, stirring with a spatula to avoid the bottom burning, for about 8-12 minutes.
3 (450g) cups three-berry blend, 1/3 (65g) cup granulated white sugar, 2 (20g) Tbsp lemon juice
Fresh Berry Swap: If you want to use fresh berries, swap them for the frozen before cooking the compote.
1 (150g) cup fresh blueberries, 1 (125g) cup fresh raspberries, 1 (150g) cup fresh strawberries
The berries should start to break down after about 10 minutes, and release their juices. If you have any larger strawberries remove them at this point and cut them into smaller pieces and return them to the compote.
In a small bowl, add the cold water and cornstarch. Whisk together until smooth.
1 1/2 Tbsp cornstarch, 2 Tbsp water
Add the cornstarch slurry to the berry compote after the compote has cooked down, and whisk constantly to avoid clumps.
Cook the compote 1-2 minutes until thick and jammy. Transfer to a bowl, and to avoid a skin forming on the top, press a piece of plastic wrap directly on top of the compote after it has cooled a few minutes. Cool the compote completely before using.
You can refrigerate the compote to speed up the cooling or if you are preparing the compote ahead.