Bring 1 inch of water to a boil in a wide skillet. Reduce the heat to medium-low so the water is barely simmering. Pour the cream into a medium heat-resistant bowl, and then place the bowl into the skillet. Heat the cream, stirring often, to 75 to 80C (175F). If you do not have a thermometer, wait until small bubbles keep trying to push up to the surface.
It will take about 15 minutes of delicate heating. Add the lemon juice and continue heating the mixture, stirring gently, until the cream curdles. Do not expect the same action as you see during ricotta cheese making. All that the whipping cream will do is become thicker, like a well-done crxe8me anglaise/ custard. It should coat the back of your wooden spoon thickly. You will see just a few clear whey streaks when you stir.
Remove the bowl from the water and let cool for about 20 minutes. Meanwhile, line a sieve with four layers of dampened cheesecloth and set it over a bowl. Transfer the mixture into the lined sieve. Do not squeeze the cheese in the cheesecloth or press on its surface (be patient, it will firm up after refrigeration time). Once cooled completely, cover with plastic wrap and refrigerate (in the sieve) overnight or up to 24 hours.
The next morning you should have thick and creamy mascarpone cheese, ready to use. Keep refrigerated and use within 3 to 4 days. Please note that this process produces very little whey.
This recipe makes about 1 1/2 cups of mascarpone cheese ( a little over 300gm approx.)