Special equipment: large sieve, fine-mesh cheesecloth
Line a large sieve with a layer of heavy-duty (fine-mesh) cheesecloth and place it over a large bowl.
Slowly bring milk, cream, and salt to a rolling boil in a 6-quart heavy pot over moderate heat, stirring occasionally to prevent scorching. Add lemon juice, then reduce heat to low and simmer, stirring constantly, until the mixture curdles, about 2 minutes.
Pour the mixture into the lined sieve and let it drain 1 hour. After discarding the liquid, chill the ricotta, covered; it will keep in the refrigerator 2 days.