Pour the heavy cream into a sealable jar or container.
Add the vinegar (or lemon juice) and stir gently, making sure the vinegar is thoroughly mixed throughout the cream.
Cover and allow the mixture to sit at room temperature for 4 hours, then place it in the refrigerator overnight before using.
Notes
Be sure to mix the vinegar thoroughly into the cream; otherwise, it won't thicken properly. For extra tang, use lemon juice or a third tablespoon of vinegar.