Combine heavy cream, cultured buttermilk, and lemon juice in a saucepan.

Heat the mixture over low heat until it reaches 85 degrees Fahrenheit.

Partially cover the cream and place it in a warm area (70-80 degrees Fahrenheit).

Allow the cream to thicken and develop a slightly tart flavor, which may take 12-24 hours.

Once thickened, transfer the creme fraiche to a covered container.

Refrigerate for up to 10 days.

Note that the creme fraiche will continue to thicken as it chills.