Combine cream and lemon juice in a glass jar. Screw on lid and shake lightly until combined.
2 cups heavy cream, 4 teaspoons lemon juice
Remove lid and use a rubber band to secure a thin piece of cloth or a napkin over the jar. This allows the mixture to breathe, while also keeping out bugs.
Set jar at room temperature for several hours or overnight until thickened. Stir and serve.
Refrigerate leftover sour cream for up to two weeks.
Notes
As soon as the ingredients are combine, you may be able to see the mixture thicken a bit as the lemon juice reacts with the high fat cream.
In low temperatures, it may take up to 24 hours to thicken. To create a higher temperature, you can set the jar on a heating pad on low, in an Instant Pot set to warm, or on a seed starting mat (!) to hasten the process.