Add the lemon juice. Continue to simmer the cream for 5 minutes at 190 to 200 degrees, stirring constantly. The cream will thicken a bit, but it won’t separate like it does when you make homemade ricotta.
Remove the cream from the stove and let it cool for 45 minutes. It will thicken as it cools.
Set a fine-mesh strainer over a large bowl. Line the strainer with several layers of cheesecloth, or my favorite, butter muslin. (I love this muslin because it’s more tightly woven than traditional cheesecloth and it can go in the washing machine.)
Pour the cream into the lined strainer and cover. Strain the mascarpone in the refrigerator for 8-12 hours, until it reaches the consistency you want. Transfer it to an airtight container for up to 5 days.