Bring the heavy cream to a low simmer in a small saucepan. Whisk to avoid scorching.
Remove from the heat. Add 1 tablespoon lemon juice to the hot cream.
Switch to a spoon if you’ve been whisking, and stir. A whisk will break up the curds you’re cooking out of the cream.
The cream will begin to form curds and thicken. Add the remaining lemon juice to the pan; stir.
Let the cream cool down. Transfer all of it to a closed container and chill it overnight.
You’ll find curds and thin whey in the morning. Line a strainer with a cheesecloth or a clean, dry kitchen towel.
Pour the cream into the lined strainer. Gently squeeze out any whey.
Let the mascarpone drip, covered, for ½ hour, then spoon it into a container with a tight lid and store in the refrigerator. Use it up soon.