Line a large sieve with a layer of heavy-duty (fine-mesh) cheesecloth or dampened paper towel (we used coffee filters) and place it over a large bowl. Slowly bring 2 quarts whole milk, 1 cup heavy cream, and 1/2 teaspoon salt to a rolling boil in a 6-quart heavy pot over moderate heat, stirring occasionally to prevent scorching. Add 3 tablespoons fresh lemon juice, reduce the heat to low, and simmer, stirring constantly, until the mixture curdles, about 2 minutes. Pour he mixture into the lined sieve ad let it drain for 1 hour. After discarding the liquid, chill the ricotta, covered; it will keep in the refrigerator for up to 2 days.