Pour 1 litre heavy cream into a medium saucepan, and clip on a thermometer. Heat it gently on medium-low heat to 185˚F (85˚C) while stirring constantly.
Once it reaches the temperature, take it off the heat and stir in 2 tablespoons lemon juice. Leave it to cool to room temperature.
Place a fine-mesh sieve over a bowl, and line it with a coffee filter or 3 layers of good-quality paper towel. Do not use cheesecloth, or the cream could completely leak through it.
Cover it with plastic wrap, and leave overnight in the fridge. The next day, you should have a strainer full of beautiful thick mascarpone cheese, and a bowl with some whey which you can save that to use in soup or smoothies.
Transfer the mascarpone to an airtight container and refrigerate for 3–5 days.
