In a clean jar, combine the heavy cream and lemon juice (or vinegar). Put on a lid and shake the mixture for 1-2 minutes until slightly thickened.
Replace the lid with a cheesecloth or a clean kitchen towel and secure with a rubber band. Let it sit at room temperature overnight to ferment.
After fermentation, stir the sour cream well. Cover with a lid and refrigerate.
Before serving, thin it with some whole milk if you would like it to have a thinner consistency.