1. Make a draining system by lining a large sieve with a muslin dish towel or a fine-mesh cheesecloth (available in many grocery stores) and place it over a large bowl.
2. In a 6-quart heavy saucepan over moderate heat, slowly bring the milk, cream, and salt to a rolling boil, stirring occasionally to prevent scorching, about 20 minutes. Add the lemon juice, then reduce the heat to low and simmer, stirring constantly, until the mixture curdles (forms clumps), about 1 to 2 minutes.
3. Pour the mixture into the sieve and let it drain for about 15 minutes, then discard the liquid (the whey). You can eat the ricotta right away or cover and chill it; it will keep in the refrigerator for 2 days.