In a medium saucepan over medium heat, combine the water, erythritol, liquid monk fruit, salt, cinnamon, and zest. Stir until the sweetener is fully dissolved.
Add the cranberries and lemon juice to the pan. Cook uncovered for about 10-15 minutes, stirring occasionally. The cranberries will burst and release their juices into the sauce. The sauce is done when it has thickened enough to coat the back of a spoon. It will thicken further as it cools.
Remove pan from heat. Adjust sweetness if needed. If desired, use an immersion blender to achieve a smooth texture or a potato masher for a chunkier sauce. Serve warm or chilled.