In a medium saucepan, mix the water with stevia and vanilla extract.
Add the cranberries. Bring to a simmer over medium heat (this should take about 3 minutes).
Lower the heat to medium-low to keep a consistent simmer. Cook, uncovered, stirring occasionally, until the cranberries are soft and starting to burst, about 10 minutes.
Remove the saucepan from the heat. Taste the sauce and decide if its level of tartness suits you or if you’d like to add more stevia. I sometimes mix in an additional 1 teaspoon of stevia at this point (which equals ⅓ cup of sugar). Cool the sauce to room temperature before serving.
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Notes
If you prefer a granulated sweetener instead of stevia, please use a powdered sweetener. A regular granulated sweetener such as erythritol will crystalize as the sauce cools down, interfering with the texture of the sauce.
Don't overcook the sauce. You want the cranberries soft, and you want some of them to burst, but not all. The goal is for some of them to retain their shape. This ensures the best texture.
You can keep this sauce in the fridge, in an airtight container, for up to 4 days. It keeps well. It tastes just as good on the fourth day as it does on the first. You can also freeze this sauce for up to three months.