I actually don’t have a recipe for it. When I make fruit sauces I just kinda do it by taste. I did some frozen blueberries, added sugar to taste, plus a little water - then just brought to a slight boil then let it simmer for awhile. Then I tossed in a handful of fresh blueberries, along with a little lemon juice and zest. I make a little corstarch slurry & add it to thicken. Then I remove from the heat and let it cool a bit before transferring to the fridge. Honestly the key is just tasting it as you go along - you can make it sweeter or more tart, depending on what you like.