Add the raspberries, sugar substitute, lemon juice, and xanthan gum to a small saucepan.
Cook over medium heat for about 10 minutes, stirring occasionally, until the berries soften and the sauce is syrupy enough to lightly coat a spoon. Mash the berries as they cook to your preferred texture.
Remove from the heat and allow the sauce to cool. It will continue to thicken as it cools.
Video
Notes
Frozen raspberries can go straight into the saucepan without thawing.
For a seedless sauce, press it through a fine-mesh strainer while warm.
The sauce continues to thicken as it cools, so don't overcook it while hot.