Place the cream in a small saucepan over a low heat. Cook gently until the cream reaches 85C/185F. If you don't have a thermometer, this is just before it begins to boil.
Take the cream off the heat just to add the lemon juice. Continue to cook for 2-3 minutes or until it has thickened and become a little gritty in texture.
Remove from the heat and pour into a muslin cloth, nut milk bag or very thin, clean and unprinted/coloured tea towel over a sieve. If, when you are straining your mascarpone, the liquid running through the sieve is not mostly clear and it looks like milk or cream (some opaque blobs are ok, but it should be predominantly a clear yellowy liquid) return all of the mascarpone back to the saucepan, cook for a minute or two, allow to sit in the saucepan off heat for five or so minutes, then try again.
Strain up to 24 hours (I put my strainer on a measuring jug, cover as best I can and pop into the fridge overnight).
Once strained, place into an airtight container and stir through a little salt, if you fancy it. Use within 3-4 days.