Bring the cream to a low simmer (but not a boil) in a large saucepan. (You want it to hit around 180F on a candy thermometer.)

Keep the cream at 180F for 3 minutes, and then mix in the lemon juice.

Simmer for 3-5 minutes more, stirring frequently. Then, remove the mixture from the heat.

Let the mixture cool completely (1 – 1 ½ hours). (You can even place it in the refrigerator for 30 minutes or so to speed up the process.)

Line a fine-mesh strainer with 4 layers of cheesecloth and put a bowl under the strainer.

Pour the cooled cream mixture into the cheesecloth. Place a plate over the strainer to act as a lid, and then place it in the refrigerator to strain overnight. (12-18 hours)

Once the mascarpone is strained and thick, use it immediately or store it in an airtight container for up to a week.